《免疫学》（英文版） Chapter 16 Hypersensitive Reactions

hypersensitive chapter 16 Reactions N IMMUNE RESPONSE MOBILIZES A BATTERY OF effector molecules that act to remove antigen by various mechanisms described in previous chap ters. Generally, these effector molecules induce a localized inflammatory response that eliminates antigen without extensively damaging the host's tissue. Under certain cir- cumstances, however, this inflammatory response can have deleterious effects, resulting in significant tissue damage or even death. This inappropriate immune response is termed hypersensitivity or allergy. Although the word hypersens A Second Exposure to Poison Oak May Result in Delayed-Type Hypersensitivity ivity implies an increased response, the response is not always heightened but may, instead, be an inappropriate im mune response to an antigen. Hypersensitive reactions may Gell and Coombs Classification develop in the course of either humoral or cell-mediated e IgE-Mediated (Type D) Hypersensitivity responses The ability of the immune system to respond inappro- a Antibody-Mediated Cytotoxic (Type Il) priately to antigenic challenge was recognized early in this Hypersensitivity century. Two French scientists, Paul Portier and Charles a Immune Complex-Mediated (Type lID Richet, investigated the problem of bathers in the Mediter- Hypersensitivity ranean reacting violently to the stings of Portuguese Man of Warjellyfish. Portier and Richet concluded that the localized Type IV or Delayed-Type Hypersensitivity(DTH) reaction of the bathers was the result of toxins To counteract this reaction, the scientists experimented with the use of isolated jellyfish toxins as vaccines. Their first attempts met with disastrous results. Portier and Richet injected dogs with the purified toxins, followed later by a booster of toxins Instead of reacting to the booster by producing antibodies against the toxins, the dogs immediately reacted with vomit- ing, diarrhea, asphyxia, and, in some instances, death. Clear- Gell and Coombs Classification the antigen Portier and Richet coined the term anaphylaxis, Several forms of hypersensitive reaction can be distin- loosely translated from Greek to mean the opposite of guished, reflecting differences in the effector molecules gen- prophylaxis, to describe this overreaction. Richet was subse- erated in the course of the reaction In immediate hypersen- quently awarded the Nobel Prize in Physiology or Medicine sitive reactions, different antibody isotypes induce different in 1913 for his work on anaphylaxis immune effector molecules. IgE antibodies, for example, bu We currently refer to anaphylactic reactions within the induce mast-cell degranulation with release of histamine Imoral branch initiated by antibody or antigen-antibody and other biologically active molecules. IgG and IgM anti- complexes as immediate hypersensitivity, because the symp- bodies, on the other hand, induce hypersensitive reactions toms are manifest within minutes or hours after a sensitized by activating complement. The effector molecules in the recipient encounters antigen. Delayed-type hypersensitiv- complement reactions are the membrane-attack complex ity (dTh) is so named in recognition of the delay of symp- and such complement split products as C3a, C4a, and C5a toms until days after exposure. This chapter examines t In delayed-type hypersensitivity reactions, the effector mechanisms and consequences of the four primary types of molecules are various cytokines secreted by activated TH hypersensitive reactions. Tc cells

■ Gell and Coombs Classification ■ IgE-Mediated (Type I) Hypersensitivity ■ Antibody-Mediated Cytotoxic (Type II) Hypersensitivity ■ Immune Complex–Mediated (Type III) Hypersensitivity ■ Type IV or Delayed-Type Hypersensitivity (DTH) A Second Exposure to Poison Oak May Result in Delayed-Type Hypersensitivity Hypersensitive Reactions A       effector molecules that act to remove antigen by various mechanisms described in previous chap￾ters. Generally, these effector molecules induce a localized inflammatory response that eliminates antigen without extensively damaging the host’s tissue. Under certain cir￾cumstances, however, this inflammatory response can have deleterious effects, resulting in significant tissue damage or even death. This inappropriate immune response is termed hypersensitivity or allergy. Although the word hypersensi￾tivity implies an increased response, the response is not always heightened but may, instead, be an inappropriate im￾mune response to an antigen. Hypersensitive reactions may develop in the course of either humoral or cell-mediated responses. The ability of the immune system to respond inappro￾priately to antigenic challenge was recognized early in this century. Two French scientists, Paul Portier and Charles Richet, investigated the problem of bathers in the Mediter￾ranean reacting violently to the stings of Portuguese Man of War jellyfish. Portier and Richet concluded that the localized reaction of the bathers was the result of toxins. To counteract this reaction, the scientists experimented with the use of isolated jellyfish toxins as vaccines. Their first attempts met with disastrous results. Portier and Richet injected dogs with the purified toxins, followed later by a booster of toxins. Instead of reacting to the booster by producing antibodies against the toxins, the dogs immediately reacted with vomit￾ing, diarrhea, asphyxia, and, in some instances, death. Clear￾ly this was an instance where the animals “overreacted” to the antigen. Portier and Richet coined the term anaphylaxis, loosely translated from Greek to mean the opposite of prophylaxis, to describe this overreaction. Richet was subse￾quently awarded the Nobel Prize in Physiology or Medicine in 1913 for his work on anaphylaxis. We currently refer to anaphylactic reactions within the humoral branch initiated by antibody or antigen-antibody complexes as immediate hypersensitivity, because the symp￾toms are manifest within minutes or hours after a sensitized recipient encounters antigen. Delayed-type hypersensitiv￾ity (DTH) is so named in recognition of the delay of symp￾toms until days after exposure. This chapter examines the mechanisms and consequences of the four primary types of hypersensitive reactions. Gell and Coombs Classification Several forms of hypersensitive reaction can be distin￾guished, reflecting differences in the effector molecules gen￾erated in the course of the reaction. In immediate hypersen￾sitive reactions, different antibody isotypes induce different immune effector molecules. IgE antibodies, for example, induce mast-cell degranulation with release of histamine and other biologically active molecules. IgG and IgM anti￾bodies, on the other hand, induce hypersensitive reactions by activating complement. The effector molecules in the complement reactions are the membrane-attack complex and such complement split products as C3a, C4a, and C5a. In delayed-type hypersensitivity reactions, the effector molecules are various cytokines secreted by activated TH or TC cells. chapter 16

62 paRt III Immune Effector Mechanisms VISUALIZING CONCEPTS ADCC Immune Sensitized TpTH for Ige A Fc receptor activation activation b Neutrophil Immune pe Ty IgE-Mediated Hypersensitivity IgGI d Cytotoxic Immune Complex-Mediated Cell-Mediated Hypersensitivity Hypersensitivity Ag induces crosslinking of Ab directed against cell surface Ag-Ab complexes deposited Sensitized THl cells release antigens meditates cell various tissues induc cytokines that activat asophils with release of destruct ion via complement complement activation ar acrophages or Tc cells which ensuing inflammatory mediate direct cellular damage response mediated by massive Typical manifestations include Typical manifestations include Typical manifestations include manifestations include systemic anaphylaxis and blood transfusion reactions, localized Arthus reaction and yd2址计h=小间山油h =三 nd graft rejection allergies, and eczema vasculitis, glomeruLinephritis, heumatoid arthritis. and systemic lupus erythematosus FIGURE The four types of hypersensitive responses As it became clear that several different immune mecha- but it is important to point out that secondary effects blur the nisms give rise to hypersensitive reactions, P G. H. Gell and boundaries between the four categories. R.R. A Coombs proposed a classification scheme in which hypersensitive reactions are divided into four types. Three types of hypersensitivity ocur within the humoral branch IgE-Mediated (Type D) Hypersensitiv IgE-mediated(type D), antibody-mediated (type ID), and im- a type I hypersensitive reaction is induced by certain types of mune complex-mediated(type lID). A fourth type of hyper- antigens referred to as allergens, and has all the hallmarks of sensitivity depends on reactions within the cell-mediated a normal humoral response. That is, an allergen induces branch, and is termed delayed-type hypersensitivity, or DTH humoral antibody response by the same mechanisms type Iv). Each type involves distinct mechanisms, cells, and described in Chapter 11 for other soluble antigens, result mediator molecules( Figure 16-1). This classification scheme in the generation of antibody-secreting plasma cells has served an important function in identifying the mecha- memory cells. What distinguishes a type I hypersensitive stic differences among various hypersensitive reactions, response from a normal humoral response is that the plasma

As it became clear that several different immune mecha￾nisms give rise to hypersensitive reactions, P. G. H. Gell and R. R. A. Coombs proposed a classification scheme in which hypersensitive reactions are divided into four types. Three types of hypersensitivity occur within the humoral branch and are mediated by antibody or antigen-antibody complexes: IgE-mediated (type I), antibody-mediated (type II), and im￾mune complex–mediated (type III). A fourth type of hyper￾sensitivity depends on reactions within the cell-mediated branch, and is termed delayed-type hypersensitivity, or DTH (type IV). Each type involves distinct mechanisms, cells, and mediator molecules (Figure 16-1). This classification scheme has served an important function in identifying the mecha￾nistic differences among various hypersensitive reactions, but it is important to point out that secondary effects blur the boundaries between the four categories. IgE-Mediated (Type I) Hypersensitivity A type I hypersensitive reaction is induced by certain types of antigens referred to as allergens, and has all the hallmarks of a normal humoral response. That is, an allergen induces a humoral antibody response by the same mechanisms as described in Chapter 11 for other soluble antigens, resulting in the generation of antibody-secreting plasma cells and memory cells. What distinguishes a type I hypersensitive response from a normal humoral response is that the plasma 362 PART III Immune Effector Mechanisms VISUALIZING CONCEPTS Type I IgE-Mediated Hypersensitivity Ag induces crosslinking of IgE bound to mast cells and basophils with release of vasoactive mediators Typical manifestations include systemic anaphylaxis and localized anaphylaxis such as hay fever, asthma, hives, food allergies, and eczema Typical manifestations include blood transfusion reactions, erythroblastosis fetalis, and autoimmune hemolytic anemia Typical manifestations include contact dermatitis, tubercular lesions and graft rejection Typical manifestations include localized Arthus reaction and generalized reactions such as serum sickness, necrotizing vasculitis, glomerulnephritis, rheumatoid arthritis, and systemic lupus erythematosus Ab directed against cell surface antigens meditates cell destruction via complement activation or ADCC Ag-Ab complexes deposited in various tissues induce complement activation and an ensuing inflammatory response mediated by massive infiltration of neutrophils Sensitized TH1 cells release cytokines that activate macrophages or TC cells which mediate direct cellular damage IgG-Mediated Cytotoxic Hypersensitivity Immune Complex-Mediated Hypersensitivity Cell-Mediated Hypersensitivity Type II Type III Type IV Allergen Allergen￾specific IgE Fc receptor for IgE Fc receptor Degranulation C3b C3b C3b Antigen Immune complex Complement activation Complement activation Immune complex C C Neutrophil Activated macrophage Cytokines Sensitized TDTH ADCC Cytotoxic cell Surface Target antigen cell FIGURE 16-1 The four types of hypersensitive responses

Hypersensitive Reactions CHAPTER 16 cells secrete IgE. This class of antibody binds with high affin- crease and remain high until the parasite is successfully ity to Fc receptors on the surface of tissue mast cells and cleared from the body. Some persons, however, may have an blood basophils. Mast cells and basophils coated by igE are abnormality called atopy, a hereditary predisposition to the said to be sensitized. A later exposure to the same allergen development of immediate hypersensitivity reactions against cross-links the membrane-bound IgE on sensitized mast cells common environmental antigens. The IgE regulatory defects and basophils, causing degranulation of these cells(figure suffered by atopic individuals allow nonparasitic antigens to 16-2). The pharmacologically active mediators released from stimulate inappropriate igE production, leading to tissue- the granules act on the surrounding tissues. The principal damaging type I hypersensitivity. The term allergen refers effects-vasodilation and smooth-muscle contraction-may specifically to nonparasitic antigens capable of stimulating be either systemic or localized, depending on the extent of type I hypersensitive responses in allergic individuals mediator release The abnormal IgE response of atopic individuals is at least partly genetic-it often runs in families. Atopic individuals have There Are Several components abnormally high levels of circulating IgE and also more tha of Type I Reactions normal numbers of circulating eosinophils. These individuals are more susceptible to allergies such as hay fever, eczema, and As depicted in Figure 16-2, several components are critical to asthma. The genetic propensity to atopic responses has been development of type I hypersensitive reactions. This section mapped to several candidate loci. One locus, on chromosome will consider these components first and then describe the 5q, is linked to a region that encodes a variety of cytokines, mechanism of degranulation including IL-3, IL-4, IL-5, IL-9, IL-13, and GM-CSE A second ALLERGENS locus, on chromosome 11q is linked to a region that encodes the chain of the high-affinity IgE receptor. It is known that inher- The majority of humans mount significant IgE responses ited atopy is multigenic and that other loci probably also are only as a defense against parasitic infections. After an indi- involved. Indeed, as information from the Human Genome vidual has been exposed to a parasite, serum igE levels in- Project is analyzed, other candidate genes may be revealed. B cel Tu cell Allergen Small blood vessel Fc receptor amines y。必 Mucous g 名 Blood platelets 9+Aller Plasma cell Sensitized mast cell Degranulation Allergen IGURE 16-2 General mechanism underlying a type I hypersens- tor ) Second exposure to the allergen leads to crosslinking of the ve reaction. Exposure to an allergen activates B cells to form IgE- bound ige, triggering the release of pharmacologically active media- secreting plasma cells. The secreted igE molecules bind to IgE- tors, vasoactive amines, from mast cells and basophils. The media specific Fc receptors on mast cells and blood basophils. ( Many mol- tors cause smooth-muscle contraction, increased vascular perme- cules of igE with various specificities can bind to the igE-Fc recep- ability, and vasodilation

cells secrete IgE. This class of antibody binds with high affin￾ity to Fc receptors on the surface of tissue mast cells and blood basophils. Mast cells and basophils coated by IgE are said to be sensitized. A later exposure to the same allergen cross-links the membrane-bound IgE on sensitized mast cells and basophils, causing degranulation of these cells (Figure 16-2). The pharmacologically active mediators released from the granules act on the surrounding tissues. The principal effects—vasodilation and smooth-muscle contraction—may be either systemic or localized, depending on the extent of mediator release. There Are Several Components of Type I Reactions As depicted in Figure 16-2, several components are critical to development of type I hypersensitive reactions. This section will consider these components first and then describe the mechanism of degranulation. ALLERGENS The majority of humans mount significant IgE responses only as a defense against parasitic infections. After an indi￾vidual has been exposed to a parasite, serum IgE levels in￾crease and remain high until the parasite is successfully cleared from the body. Some persons, however, may have an abnormality called atopy, a hereditary predisposition to the development of immediate hypersensitivity reactions against common environmental antigens. The IgE regulatory defects suffered by atopic individuals allow nonparasitic antigens to stimulate inappropriate IgE production, leading to tissue￾damaging type I hypersensitivity. The term allergen refers specifically to nonparasitic antigens capable of stimulating type I hypersensitive responses in allergic individuals. The abnormal IgE response of atopic individuals is at least partly genetic—it often runs in families.Atopic individuals have abnormally high levels of circulating IgE and also more than normal numbers of circulating eosinophils. These individuals are more susceptible to allergies such as hay fever, eczema, and asthma. The genetic propensity to atopic responses has been mapped to several candidate loci. One locus, on chromosome 5q, is linked to a region that encodes a variety of cytokines, including IL-3, IL-4, IL-5, IL-9, IL-13, and GM-CSF. A second locus,on chromosome 11q,is linked to a region that encodes the chain of the high-affinity IgE receptor. It is known that inher￾ited atopy is multigenic and that other loci probably also are involved. Indeed, as information from the Human Genome Project is analyzed, other candidate genes may be revealed. Hypersensitive Reactions CHAPTER 16 363 Memory cell Plasma cell Sensitized mast cell B cell TH cell Allergen CD4 IL-4 Allergen￾specific IgE Fc receptor for IgE + Allergen Allergen Eosinophil Sensory–nerve endings Blood platelets Mucous gland Vasoactive amines Small blood vessel Smooth muscle cell Degranulation FIGURE 16-2 General mechanism underlying a type I hypersensi￾tive reaction. Exposure to an allergen activates B cells to form IgE￾secreting plasma cells. The secreted IgE molecules bind to IgE￾specific Fc receptors on mast cells and blood basophils. (Many mol￾ecules of IgE with various specificities can bind to the IgE-Fc recep￾tor.) Second exposure to the allergen leads to crosslinking of the bound IgE, triggering the release of pharmacologically active media￾tors, vasoactive amines, from mast cells and basophils. The media￾tors cause smooth-muscle contraction, increased vascular perme￾ability, and vasodilation.

364 paRI I Immune Effector mechanisms TABLE 16-7 Common allergens associated bergens are small proteins or protein-bound substance with type I hypersensitivity aving a molecular weight between 15,000 and 40,000,at tempts to identify some common chemical property of these Proteins Foods antigens have failed. It a cons reIc Nuts quence of a complex series of interactions involving not only the allergen but also the dose the sens Plant pollens Peas, beans times an adjuvant, and-most important, as noted above- Rye grass the genetic constitution of the recipient. Insect products REAGINIC ANTIBODY(IGE) As described in Chapter 4, the existence of a human serum Drugs factor that reacts with allergens was first demonstrated by Penicillin Cockroach calyx K Prausnitz and h. Kustner in 1921. The local wheal and Sulfonamides Dust mites flare response that occurs when an allergen is injected into Local anesthetics sensitized individual is callled the p-k reaction because the Salicylates Mold spores serum components responsible for the P-K reaction dis Animal hair and dander played specificity for allergen, they were assumed to be anti bodies, but the nature of these p-K antibodies or reagins, was not demonstrated for many years Experiments conducted by K and T Ishizaka in the mid- 1960s showed that the biological activity of reaginic antibody Most allergic IgE responses occur on mucous membrane in a p-K test could be neutralized by rabbit antiserum against surfaces in response to allergens that enter the body by either whole atopic human sera but not by rabbit antiserum specific inhalation or ingestion. Of the common allergens listed in for the four human immunoglobulin classes known at that Table 16-1, few have been purified and characterized. Those time (IgA, IgG, IgM, and IgD)(Table 16-2). In addition, when that have include the allergens from rye grass pollen, ragweed rabbits were immunized with sera from ragweed-sensitive pollen, codfish, birch pollen, timothy grass pollen, and bee individuals, the rabbit antiserum could inhibit (neutralize)a venom. Each of these allergens has been shown to be a multi- positive ragweed P-K test even after precipitation of the rabbit antigenic system that contains a number of allergenic com- antibodies specific for the human IgG, IgA, IgM, and lgd iso- ponents. Ragweed pollen, a major allergen in the United types. The Ishizakas called this new isotype lgE in reference States, is a case in point. It has been reported that a square the E antigen of ragweed that they used to characterize it. mile of ragweed yields 16 tons of pollen in a single season Serum IgE levels in normal individuals fall within the Indeed, all regions of the United States are plagued by rag- range of 0. 1-0.4 ug/ml; even the most severely allergic indi- weed pollen as well as pollen from trees indigenous to the viduals rarely have lgE levels greater than 1 ug/ml. These low region. The pollen particles are inhaled, and their tough levels made physiochemical studies of Ige difficult; it was not outer wall is dissolved by enzymes in the mucous secretions, until the discovery of an IgE myeloma by S G O Johansson releasing the allergenic substances. Chemical fractionation of and H. Bennich in 1967 that extensive chemical analysis of ragweed has revealed a variety of substances, most of which IgE could be undertaken IgE was found to be composed of are not allergenic but are capable of eliciting an igM or igG two heavy e and two light chains with a combined molecular response Of the five fractions that are allergenic(i., able to weight of 190,000. The higher molecular weight as compared induce an IgE response), two evoke allergenic reactions in with IgG (150,000)is due to the presence of an additional about 95% of ragweed-sensitive individuals and are called constant-region domain(see Figure 4-13). This additional major allergens; these are designated the E and K fractions. domain( CH4)contributes to an altered conformation of the The other three, called Ra3, Ra4, and Ra5, are minor allergens Fc portion of the molecule that enables it to bind to glyco. that induce an allergic response in only 20% to 30% of sensi- protein receptors on the surface of basophils and mast cells. tive subjects. lthough the half-life of igE in the serum is only 2-3 days b Why are some pollens(e.g, ragweed) highly allergenic, once IgE has been bound to its receptor on mast cells and hereas other equally abundant pollens (e.g, nettle)are basophils, it is stable in that state for a number of weeks rarely allergenic? No single physicochemical property seem to distinguish the highly allergenic e and K fractions of rag- MAST CELLS AND BASOPHILS weed from the less allergenic Ra3, Ra4, and Ra5 fractions and The cells that bind lgE were identified by incubating human from the nonallergenic fractions. Rather, allergens as a group leukocytes and tissue cells with either I-labeled IgE mye appear to possess diverse properties. Some allergens, includ- loma protein or I-labeled anti-IgE. In both cases, autoradi- ing foreign serum and egg albumin, are potent antigens; oth- ography revealed that the labeled probe bound with high ers, such as plant pollens, are weak antigens. Although most affinity to blood basophils and tissue mast cells. Basophils are

Most allergic IgE responses occur on mucous membrane surfaces in response to allergens that enter the body by either inhalation or ingestion. Of the common allergens listed in Table 16-1, few have been purified and characterized. Those that have include the allergens from rye grass pollen, ragweed pollen, codfish, birch pollen, timothy grass pollen, and bee venom. Each of these allergens has been shown to be a multi￾antigenic system that contains a number of allergenic com￾ponents. Ragweed pollen, a major allergen in the United States, is a case in point. It has been reported that a square mile of ragweed yields 16 tons of pollen in a single season. Indeed, all regions of the United States are plagued by rag￾weed pollen as well as pollen from trees indigenous to the region. The pollen particles are inhaled, and their tough outer wall is dissolved by enzymes in the mucous secretions, releasing the allergenic substances. Chemical fractionation of ragweed has revealed a variety of substances, most of which are not allergenic but are capable of eliciting an IgM or IgG response. Of the five fractions that are allergenic (i.e., able to induce an IgE response), two evoke allergenic reactions in about 95% of ragweed-sensitive individuals and are called major allergens; these are designated the E and K fractions. The other three, called Ra3, Ra4, and Ra5, are minor allergens that induce an allergic response in only 20% to 30% of sensi￾tive subjects. Why are some pollens (e.g., ragweed) highly allergenic, whereas other equally abundant pollens (e.g., nettle) are rarely allergenic? No single physicochemical property seems to distinguish the highly allergenic E and K fractions of rag￾weed from the less allergenic Ra3, Ra4, and Ra5 fractions and from the nonallergenic fractions. Rather, allergens as a group appear to possess diverse properties. Some allergens, includ￾ing foreign serum and egg albumin, are potent antigens; oth￾ers, such as plant pollens, are weak antigens. Although most allergens are small proteins or protein-bound substances having a molecular weight between 15,000 and 40,000, at￾tempts to identify some common chemical property of these antigens have failed. It appears that allergenicity is a conse￾quence of a complex series of interactions involving not only the allergen but also the dose, the sensitizing route, some￾times an adjuvant, and—most important, as noted above— the genetic constitution of the recipient. REAGINIC ANTIBODY (IGE) As described in Chapter 4, the existence of a human serum factor that reacts with allergens was first demonstrated by K. Prausnitz and H. Kustner in 1921. The local wheal and flare response that occurs when an allergen is injected into a sensitized individual is called the P-K reaction. Because the serum components responsible for the P-K reaction dis￾played specificity for allergen, they were assumed to be anti￾bodies, but the nature of these P-K antibodies, or reagins, was not demonstrated for many years. Experiments conducted by K. and T. Ishizaka in the mid- 1960s showed that the biological activity of reaginic antibody in a P-K test could be neutralized by rabbit antiserum against whole atopic human sera but not by rabbit antiserum specific for the four human immunoglobulin classes known at that time (IgA, IgG, IgM, and IgD) (Table 16-2). In addition, when rabbits were immunized with sera from ragweed-sensitive individuals, the rabbit antiserum could inhibit (neutralize) a positive ragweed P-K test even after precipitation of the rabbit antibodies specific for the human IgG, IgA, IgM, and IgD iso￾types. The Ishizakas called this new isotype IgE in reference to the E antigen of ragweed that they used to characterize it. Serum IgE levels in normal individuals fall within the range of 0.1–0.4 g/ml; even the most severely allergic indi￾viduals rarely have IgE levels greater than 1 g/ml. These low levels made physiochemical studies of IgE difficult; it was not until the discovery of an IgE myeloma by S. G. O. Johansson and H. Bennich in 1967 that extensive chemical analysis of IgE could be undertaken. IgE was found to be composed of two heavy  and two light chains with a combined molecular weight of 190,000. The higher molecular weight as compared with IgG (150,000) is due to the presence of an additional constant-region domain (see Figure 4-13). This additional domain (CH4) contributes to an altered conformation of the Fc portion of the molecule that enables it to bind to glyco￾protein receptors on the surface of basophils and mast cells. Although the half-life of IgE in the serum is only 2–3 days, once IgE has been bound to its receptor on mast cells and basophils, it is stable in that state for a number of weeks. MAST CELLS AND BASOPHILS The cells that bind IgE were identified by incubating human leukocytes and tissue cells with either 125I-labeled IgE mye￾loma protein or 125I-labeled anti-IgE. In both cases, autoradi￾ography revealed that the labeled probe bound with high affinity to blood basophils and tissue mast cells. Basophils are 364 PART III Immune Effector Mechanisms TABLE 16-1 Common allergens associated with type I hypersensitivity Proteins Foods Foreign serum Nuts Vaccines Seafood Eggs Plant pollens Peas, beans Rye grass Milk Ragweed Timothy grass Insect products Birch trees Bee venom Wasp venom Drugs Ant venom Penicillin Cockroach calyx Sulfonamides Dust mites Local anesthetics Salicylates Mold spores Animal hair and dander

Hypersensitive Reactions CHAPTER 16 365 TABLE 16-2 Identification of IgE based on reactivity of atopic serum in P-K test Treatment Allergen added P-K reaction at skin site A None antiserum to human atopic serum Rabbit antiserum to human igM, igG, IgA, and IgD Serum from an atopic individual was injected into rabbits to produce antiserum against human atopic serum. When this antiserum was reacted with human atopic serum, it neutralized the p-k reaction. SOURCE: Based on K Ishizaka and T Ishizaka, 1967, J Immunol. 99: 1187. granulocytes that circulate in the blood of most vertebrates; mm. Electron micrographs of mast cells reveal numerous in humans, they account for 0.5%-1.0% of the circulating membrane-bounded granules distributed throughout the white blood cells. Their granulated cytoplasm stains with cytoplasm, which, like those in basophils, contain pharmaco- basic dyes, hence the name basophil. Electron microscopy re- logically active mediators(Figure 16-3). After activation, these veals a multilobed nucleus, few mitochondria, numerous mediators are released from the granules, resulting in the clin- glycogen granules, and electron-dense membrane-bound ical manifestations of the type I hypersensitive reaction. granules scattered throughout the cytoplasm that contain Mast cell populations in different anatomic sites differ sig pharmacologically active mediators(see Figure 2-10c) nificantly in the types and amounts of allergic mediators they Mast-cell precursors are formed in the bone marrow dur- contain and in their sensitivity to activating sti ing hematopoiesis and are carried to virtually all vascularized cytokines. Mast cells also secrete a large variety of cytokine peripheral tissues, where they differentiate into mature cells. that affect a broad spectrum of physiologic, immunologic, Mast cells are found throughout connective tissue, particu- and pathologic processes(see Table 12-1) larly near blood and lymphatic vessels. Some tissues, includ- ing the skin and mucous membrane surfaces of the respira- IgE-BINDING FC RECEPTORS tory and gastrointestinal tracts, contain high concentrations The reaginic activity of igE depends on its ability to bind to a of mast cells; skin, for example, contains 10,000 mast cells receptor specific for the Fc region of the e heavy chain. Two FIGURE 16-3(a)Electron micrograph of a typical mast cell reveals membrane of a mast cell. (c)Granule releasing its contents(towards numerous electron-dense membrane- bounded granules prior to top left) during degranulation. / From S. Burwen and B. Satir, 1977, degranulation. (b) Close-up of intact granule underlying the plasma J Cell Biol. 73: 662. 1

granulocytes that circulate in the blood of most vertebrates; in humans, they account for 0.5%–1.0% of the circulating white blood cells. Their granulated cytoplasm stains with basic dyes, hence the name basophil. Electron microscopy re￾veals a multilobed nucleus, few mitochondria, numerous glycogen granules, and electron-dense membrane-bound granules scattered throughout the cytoplasm that contain pharmacologically active mediators (see Figure 2-10c). Mast-cell precursors are formed in the bone marrow dur￾ing hematopoiesis and are carried to virtually all vascularized peripheral tissues, where they differentiate into mature cells. Mast cells are found throughout connective tissue, particu￾larly near blood and lymphatic vessels. Some tissues, includ￾ing the skin and mucous membrane surfaces of the respira￾tory and gastrointestinal tracts, contain high concentrations of mast cells; skin, for example, contains 10,000 mast cells per mm3 . Electron micrographs of mast cells reveal numerous membrane-bounded granules distributed throughout the cytoplasm, which, like those in basophils, contain pharmaco￾logically active mediators (Figure 16-3). After activation, these mediators are released from the granules, resulting in the clin￾ical manifestations of the type I hypersensitive reaction. Mast cell populations in different anatomic sites differ sig￾nificantly in the types and amounts of allergic mediators they contain and in their sensitivity to activating stimuli and cytokines. Mast cells also secrete a large variety of cytokines that affect a broad spectrum of physiologic, immunologic, and pathologic processes (see Table 12-1). IgE-BINDING Fc RECEPTORS The reaginic activity of IgE depends on its ability to bind to a receptor specific for the Fc region of the  heavy chain. Two Hypersensitive Reactions CHAPTER 16 365 TABLE 16-2 Identification of IgE based on reactivity of atopic serum in P-K test Serum Treatment Allergen added P-K reaction at skin site Atopic None – – Atopic None + + Nonatopic None + – Atopic Rabbit antiserum to human atopic serum* + – Atopic Rabbit antiserum to human IgM, IgG, IgA, and IgD† + + *Serum from an atopic individual was injected into rabbits to produce antiserum against human atopic serum. When this antiserum was reacted with human atopic serum, it neutralized the P-K reaction. † Serum from an atopic individual was reacted with rabbit antiserum to the known classes of human antibody (IgM, IgA, IgG, and IgD) to remove these isotypes from the atopic serum. The treated atopic serum continued to give a positive P-K reaction, indicating that a new immunoglobulin isotype was responsible for this reactivity. SOURCE: Based on K. Ishizaka and T. Ishizaka, 1967, J. Immunol. 99:1187. (a) (b) (c) FIGURE 16-3 (a) Electron micrograph of a typical mast cell reveals numerous electron-dense membrane-bounded granules prior to degranulation. (b) Close-up of intact granule underlying the plasma membrane of a mast cell. (c) Granule releasing its contents (towards top left) during degranulation. [From S. Burwen and B. Satir, 1977, J. Cell Biol. 73:662.]

366 paRI I Immune Effector mechanisms (a)FcERI: High-affinity IgE receptor Low-affinity IgE receptor Soluble(s CD23 ig-lik COOH Extracellular H2N NH, y lavage aPlasma &membrane Cytoplasm COOHCOOH ∠mAM COOHCOOH FIGURE 16-4 Schematic diagrams of the high-affinity FCERI and CD3 complex of the T-cell receptor.(b)The low-affinity receptor is un- low-affinity FCERll receptors that bind the Fc region of igE (a) Each y usual because it is oriented in the membrane with its NH2-terminus chain of the high-affinity receptor contains an ITAM, a motif also pre- directed toward the cell interior and its COoH-terminus directed to- sent in the Ig-a/Ig-B heterodimer of the B-cell receptor and in the ward the extracellular space classes of fcer been identified, designated FcERI and fceril, brane receptors that have this motif are CD3 and the asso- which are expressed by different cell types and differ by 1000- ciated s chains of the T-cell receptor complex (see Figure fold in their affinity for igE 10-10)and the Ig-a/Ig-B chains associated with membran immunoglobulin on B cells(see Figure 11-7). The ITAM HIGH-AFFINITY RECEPTOR(FCeRI) Mast cells and baso- motif on these three receptors interacts with protein tyrosine phils express FceRl, which binds IgE with a high affinity(Kp kinases to transduce an activating signal to the cell. aller 1-2 X 10 M). The high affinity of this receptor enables mediated crosslinkage of the bound ige results in aggrega- it to bind ige despite the low serum concentration of Ige tion of the FceRI receptors and rapid tyrosine phosphoryla (1 x10-7)Between 40,000 and 90,000 FceRI molecules have tion, which initiates the process of mast-cell degranulation been shown to be present on a human basopl The role of FcERI in anaphylaxis is confirmed by experiments The FcERI receptor contains four polypeptide chains: an conducted in mice that lack FcERI. These mice have normal c and a B chain and two identical disulfide -linked y chains levels of mast cells but are resistant to localized and systemic (Figure 16-4a). The external region of the a chain contains anaphylaxis two domains of 90 amino acids that are homologous with the immunoglobulin-fold structure, placing the molecule in the LOW-AFFINITY RECEPTOR(FCeRII) The other IgE recep- immunoglobulin superfamily (see Figure 4-19). FcERI inter- tor, designated FcERIl (or CD23), is specific for the CH3/ acts with the CH3/CH3 and CH4/CH4 domains of the ige Ch3 domain of ige and has a lower affinity for ige (Kp molecule via the two Ig-like domains of the a chain. The p 1x 10-M)than does FcERI (Figure 16-4b). The FcErll chain spans the plasma membrane four times and is thought receptor appears to play a variety of roles in regulating the to link the a chain to the y homodimer. The disulfide-linked intensity of the igE response. Allergen crosslinkage of igE y chains extend a considerable distance into the cytoplasm. bound to FceRII has been shown to activate B cells, alveolar Each y chain has a conserved sequence in its cytosolic do- macrophages, and eosinophils. When this receptor is blocked main known as an immunoreceptor tyrosine-based activa- with monoclonal antibodies, IgE secretion by b cells is tion motif (ITAM). As described earlier, two other mem- diminished. a soluble form of FcERIl (or sCD23), which is

classes of FcR been identified, designated FcRI and FcRII, which are expressed by different cell types and differ by 1000- fold in their affinity for IgE. HIGH-AFFINITY RECEPTOR (FCRI) Mast cells and baso￾phils express FcRI, which binds IgE with a high affinity (KD = 1–2  10–9 M). The high affinity of this receptor enables it to bind IgE despite the low serum concentration of IgE (1  10–7). Between 40,000 and 90,000 FcRI molecules have been shown to be present on a human basophil. The FcRI receptor contains four polypeptide chains: an  and a chain and two identical disulfide-linked  chains (Figure 16-4a). The external region of the  chain contains two domains of 90 amino acids that are homologous with the immunoglobulin-fold structure, placing the molecule in the immunoglobulin superfamily (see Figure 4-19). FcRI inter￾acts with the CH3/CH3 and CH4/CH4 domains of the IgE molecule via the two Ig-like domains of the  chain. The chain spans the plasma membrane four times and is thought to link the  chain to the  homodimer. The disulfide-linked  chains extend a considerable distance into the cytoplasm. Each  chain has a conserved sequence in its cytosolic do￾main known as an immunoreceptor tyrosine-based activa￾tion motif (ITAM). As described earlier, two other mem￾brane receptors that have this motif are CD3 and the asso￾ciated  chains of the T-cell receptor complex (see Figure 10-10) and the Ig-/Ig- chains associated with membrane immunoglobulin on B cells (see Figure 11-7). The ITAM motif on these three receptors interacts with protein tyrosine kinases to transduce an activating signal to the cell. Allergen￾mediated crosslinkage of the bound IgE results in aggrega￾tion of the FcRI receptors and rapid tyrosine phosphoryla￾tion, which initiates the process of mast-cell degranulation. The role of FcRI in anaphylaxis is confirmed by experiments conducted in mice that lack FcRI. These mice have normal levels of mast cells but are resistant to localized and systemic anaphylaxis. LOW-AFFINITY RECEPTOR (FCRII) The other IgE recep￾tor, designated FcRII (or CD23), is specific for the CH3/ CH3 domain of IgE and has a lower affinity for IgE (KD = 1  10–6M) than does FcRI (Figure 16-4b). The FcRII receptor appears to play a variety of roles in regulating the intensity of the IgE response. Allergen crosslinkage of IgE bound to FcRII has been shown to activate B cells, alveolar macrophages, and eosinophils. When this receptor is blocked with monoclonal antibodies, IgE secretion by B cells is diminished. A soluble form of FcRII (or sCD23), which is 366 PART III Immune Effector Mechanisms NH2 Ig-like domains Extracellular space Plasma membrane Cytoplasm ITAM S S COOH COOH COOH COOH NH2 α β S S γ γ H2N NH2 S S NH2 Soluble CD23 S S S S S S COOH Proteolytic cleavage (a) FcεRI: High-affinity IgE receptor (b) FcεRII (CD23): Low-affinity IgE receptor FIGURE 16-4 Schematic diagrams of the high-affinity FcRI and low-affinity FcRII receptors that bind the Fc region of IgE. (a) Each  chain of the high-affinity receptor contains an ITAM, a motif also pre￾sent in the Ig-/Ig- heterodimer of the B-cell receptor and in the CD3 complex of the T-cell receptor. (b) The low-affinity receptor is un￾usual because it is oriented in the membrane with its NH2-terminus directed toward the cell interior and its COOH-terminus directed to￾ward the extracellular space.

Hypersensitive Reactions CHAPTER 16 generated by autoproteolysis of the membrane receptor, has (a) Allergen crosslinkage of (c)Chemical crosslinkage ingly, atopic individuals have higher levels of CD23 on their Ab nd lg been shown to enhance igE production by B cells. Interest- Crosslinking chemical s andn macrophages and higher levels of scD23 in their serum than do nonatopic individuals. IgE Crosslinkage Initiates Degranulation F The biochemical events that mediate degranulation of mast 5 cells and blood basophils have many features in common Mast cell For simplicity, this section presents a general overview of (d)Crosslinkage of IgE mast-cell degranulation mechanisms without calling atten tion to the slight differences between mast cells and baso b)Antibody crosslinkage receptors by of IgE phils. Although mast-cell degranulation generally is initiated Anti-receptor by allergen crosslinkage of bound igE, a number of other stimuli can also initiate the process, including the anaphyla toxins(C3a, C4a, and C5a)and various drugs. This section focuses on the biochemical events that follow allergen crosslinkage of bound igE. RECEPTOR CROSSLINKAGE (e) Enhanced Ca2+ influx IgE-mediated degranulation begins when an allergen cross- Anti-idiotype Ab tha links ige that is bound (fixed) to the Fc receptor on the sur- increases membrane face of a mast cell or basophil. In itself, the binding of igE to permeability to Ca2+ FcERI apparently has no effect on a target cell. It is only after Ca2+ allergen crosslinks the fixed igE-receptor complex that de- Ionophore granulation proceeds. The importance of crosslinkage is in- dicated by the inability of monovalent allergens, which can- not crosslink the fixed IgE, to trigger degranulation. granulation is crosslinkage of two or more FceRI mole- FIGURE 16-5 Schematic diagrams of mechanisms that can trigger cules--with or without bound IgE. Although crosslinkage is degranulation of mast cells. Note that mechanisms(b)and (c)do not normally effected by the interaction of fixed IgE with diva- require allergen: mechanisms (d)and (e)require neither allergen nor lent or multivalent allergen, it also can be effected by a vari- gE and mechanism(e)does not even require receptor crosslinkage ty of experimental means that bypass the need for allergen and in some cases even for IgE (Figure 16-5) Intracellular Events Also Regulate last-Cell Degranulation intracellular stores in the endoplasmic reticulum(see Figure 16-6). The Ca increase eventually leads to the formation of The cytoplasmic domains of the B and y chains of fceRI are arachidonic acid, which is converted into two classes of age of the FcERI receptors activates the associated PTKs. ure 16-6). The increase of Ca2+also promotes the assembly resulting in the phosphorylation of tyrosines within the of microtubules and the contraction of microfilaments, both ITAMs of the y subunit as well as phosphorylation of resi- of which are necessary for the movement of granules to the dues on the p subunit and on phospholipase C. These phos- plasma membrane. The importance of the Ca increase in phorylation events induce the production of a number of mast-cell degranulation is highlighted by the use of drugs second messengers that mediate the process of degranulation such as disodium cromoglycate(cromolyn sodium), that (Figure 16-6) block this influx as a treatment for allergies Within 15 s after crosslinkage of FceRl, methylation of Concomitant with phospholipid methylation and Ca in various membrane phospholipids is observed, resulting in an crease, there is a transient increase in the activity of membrane- increase in membrane fluidity and the formation of Ca+ bound adenylate cyclase, with a rapid peak of its reaction prod channels. An increase of Ca+ reaches a peak within 2 min of uct, cyclic adenosine monophosphate( cAMP), reached about FcERI crosslinkage( Figure 16-7). This increase is due both to 1 min after crosslinkage of FcERI(see Figure 16-7). The effects of the uptake of extracellular Ca and to a release of Ca* from cAMP are exerted through the activation of cAMP-dependent

generated by autoproteolysis of the membrane receptor, has been shown to enhance IgE production by B cells. Interest￾ingly, atopic individuals have higher levels of CD23 on their lymphocytes and macrophages and higher levels of sCD23 in their serum than do nonatopic individuals. IgE Crosslinkage Initiates Degranulation The biochemical events that mediate degranulation of mast cells and blood basophils have many features in common. For simplicity, this section presents a general overview of mast-cell degranulation mechanisms without calling atten￾tion to the slight differences between mast cells and baso￾phils. Although mast-cell degranulation generally is initiated by allergen crosslinkage of bound IgE, a number of other stimuli can also initiate the process, including the anaphyla￾toxins (C3a, C4a, and C5a) and various drugs. This section focuses on the biochemical events that follow allergen crosslinkage of bound IgE. RECEPTOR CROSSLINKAGE IgE-mediated degranulation begins when an allergen cross￾links IgE that is bound (fixed) to the Fc receptor on the sur￾face of a mast cell or basophil. In itself, the binding of IgE to FcRI apparently has no effect on a target cell. It is only after allergen crosslinks the fixed IgE-receptor complex that de￾granulation proceeds. The importance of crosslinkage is in￾dicated by the inability of monovalent allergens, which can￾not crosslink the fixed IgE, to trigger degranulation. Experiments have revealed that the essential step in de￾granulation is crosslinkage of two or more FcRI mole￾cules—with or without bound IgE. Although crosslinkage is normally effected by the interaction of fixed IgE with diva￾lent or multivalent allergen, it also can be effected by a vari￾ety of experimental means that bypass the need for allergen and in some cases even for IgE (Figure 16-5). Intracellular Events Also Regulate Mast-Cell Degranulation The cytoplasmic domains of the and  chains of FcRI are associated with protein tyrosine kinases (PTKs). Crosslink￾age of the FcRI receptors activates the associated PTKs, resulting in the phosphorylation of tyrosines within the ITAMs of the  subunit as well as phosphorylation of resi￾dues on the subunit and on phospholipase C. These phos￾phorylation events induce the production of a number of second messengers that mediate the process of degranulation (Figure 16-6). Within 15 s after crosslinkage of FcRI, methylation of various membrane phospholipids is observed, resulting in an increase in membrane fluidity and the formation of Ca2+ channels. An increase of Ca2+ reaches a peak within 2 min of FcRI crosslinkage (Figure 16-7). This increase is due both to the uptake of extracellular Ca2+ and to a release of Ca2+ from intracellular stores in the endoplasmic reticulum (see Figure 16-6). The Ca2+ increase eventually leads to the formation of arachidonic acid, which is converted into two classes of potent mediators: prostaglandins and leukotrienes(see Fig￾ure 16-6). The increase of Ca2+ also promotes the assembly of microtubules and the contraction of microfilaments, both of which are necessary for the movement of granules to the plasma membrane. The importance of the Ca2+ increase in mast-cell degranulation is highlighted by the use of drugs, such as disodium cromoglycate (cromolyn sodium), that block this influx as a treatment for allergies. Concomitant with phospholipid methylation and Ca2+ in￾crease, there is a transient increase in the activity of membrane￾bound adenylate cyclase, with a rapid peak of its reaction prod￾uct, cyclic adenosine monophosphate (cAMP), reached about 1 min after crosslinkage of FcRI (see Figure 16-7).The effects of cAMP are exerted through the activation of cAMP-dependent Hypersensitive Reactions CHAPTER 16 367 (a) Allergen crosslinkage of cell-bound IgE (b) Antibody crosslinkage of IgE (c) Chemical crosslinkage of IgE (d) Crosslinkage of IgE receptors by anti-receptor antibody (e) Enhanced Ca2+ influx by ionophore that increases membrane permeability to Ca2+ IgE Fc receptor IgE Allergen Mast cell Anti-isotype Ab Anti-idiotype Ab Crosslinking chemical Anti-receptor Ab Ionophore Ca2+ FIGURE 16-5 Schematic diagrams of mechanisms that can trigger degranulation of mast cells. Note that mechanisms (b) and (c) do not require allergen; mechanisms (d) and (e) require neither allergen nor IgE; and mechanism (e) does not even require receptor crosslinkage.

368 paRI I Immune Effector mechanisms FceRI FCeRI ③ PMT (MTIPE ATP cAMP( transient个 Protein kinase⑥ Protein kinase inactive Swollen.么 Mediators (e.g, histamine) Arachidonic acid doplasmic reticulum Leukotriene A,Prostaglandin D? LTB4 LTCA LTD4 SRS-A LTEA Secretion Secretion FIGURE 16-6 Diagrammatic overview of biochemical events in face of the plasma membrane causes an increase in membrane fluidity mast-cell activation and degranulation. Allergen crosslinkage of bound and facilitates the formation of Ca+ channels. The resulting influx of IgE results in FCERI aggregation and activation of protein tyrosine ki- Ca activates phospholipase A2, which promotes the breakdown of nase(PTk).(1)PTK then phosphorylates phospholipase C, which con- PC into lysophosphatidylcholine(lyso PC)and arachidonic acid verts phosphatidylinositol-4, 5 bisphosphate(PIP2)into diacylglycerol(5) Arachidonic acid is converted into potent mediators: the leuko- (DAG)and inositol triphosphate(IP3).(2)DAG activates protein ki- trienes and prostaglandin D. (6)FceRI crosslinkage also activates the nase C(PKC), which with Ca* is necessary for microtubular assembly membrane adenylate cyclase, leading to a transient increase of CAMP and the fusion of the granules with the plasma membrane. IP3 is a po- within 15 S. A later drop in cAMP levels is mediated by protein kinase tent mobilizer of intracellular Castores. (3)Crosslinkage of FcERI also and is required for degranulation to proceed. (7)cAMP-dependent pro- ctivates an enzyme that converts phosphatidylserine(PS)into phos. tein kinases are thought to phosphorylate the granule-membrane pro- phatidylethanolamine(PE). Eventually, PE is methylated to form phos- teins, thereby changing the permeability of the granules to water and phatidylcholine(PC)by the phospholipid methyl transferase enzymes I Ca. The consequent swelling of the granules facilitates fusion with the and ll(PMT I and Ii). (4)The accumulation of PC on the exterior sur- plasma membrane and release of the mediators protein kinases, which phosphorylate proteins on the granule Several Pharmacologic Agents Mediate membrane, thereby changing the permeability of the granules Type I Reactions of the granules facilitates their fusion with the plasma mem- The clinical manifestations of type I hypersensitive reactions brane,releasing their contents. The increase in cAMP is tran- are related to the biological effects of the mediators released sient and is followed by a drop in camP to levels below base- during mast-cell or basophil degranulation. These mediators line(see Figure 16-7). This drop in cAMP appears to be are pharmacologically active agents that act on local tissues necessary for degranulation to proceed; when cAMP level as well as on populations of secondary effector cells, includ increased by certain drugs, the degranulation process is ing eosinophils, neutrophils, T lymphocytes, monocytes, and blocked. Several of these drugs are given to treat allergic disor- platelets. The mediators thus amplifying termin ders and are considered later in this section effector mechanism, much as the complement system sery

protein kinases, which phosphorylate proteins on the granule membrane, thereby changing the permeability of the granules to water and Ca2+ (see Figure 16-6). The consequent swelling of the granules facilitates their fusion with the plasma mem￾brane, releasing their contents. The increase in cAMP is tran￾sient and is followed by a drop in cAMP to levels below base￾line (see Figure 16-7). This drop in cAMP appears to be necessary for degranulation to proceed; when cAMP levels are increased by certain drugs, the degranulation process is blocked. Several of these drugs are given to treat allergic disor￾ders and are considered later in this section. Several Pharmacologic Agents Mediate Type I Reactions The clinical manifestations of type I hypersensitive reactions are related to the biological effects of the mediators released during mast-cell or basophil degranulation. These mediators are pharmacologically active agents that act on local tissues as well as on populations of secondary effector cells, includ￾ing eosinophils, neutrophils, T lymphocytes, monocytes, and platelets. The mediators thus serve as an amplifying terminal effector mechanism, much as the complement system serves 368 PART III Immune Effector Mechanisms Swollen granule Allergen IgE Adenylate cyclase PMT II Phospho￾lipase C PKC PKC S S S S PIP2 DAG Active Inactive Ca2+ Ca2+ Ca2+ cAMP (transient) ATP Protein kinase inactive Protein kinase active IP3 Endoplasmic reticulum PC PE PS PMT I Lyso PC Phospho￾Degranulation Fusogens Microtubules and microfilaments Arachidonic acid Ca2+ Ca2+ Granule Prostaglandin D2 (PGD2) Leukotriene A4 LTB4 LTC4 LTD4 LTE4 SRS-A Secretion Secretion lipase A2 Mediators (e.g., histamine) PTK PTK PTK 1 2 6 3 4 7 5 PKC FCεRI FCεRI FIGURE 16-6 Diagrammatic overview of biochemical events in mast-cell activation and degranulation. Allergen crosslinkage of bound IgE results in FcRI aggregation and activation of protein tyrosine ki￾nase (PTK). (1) PTK then phosphorylates phospholipase C, which con￾verts phosphatidylinositol-4,5 bisphosphate (PIP2) into diacylglycerol (DAG) and inositol triphosphate (IP3). (2) DAG activates protein ki￾nase C (PKC), which with Ca2+ is necessary for microtubular assembly and the fusion of the granules with the plasma membrane. IP3 is a po￾tent mobilizer of intracellular Ca2+ stores. (3) Crosslinkage of FcRI also activates an enzyme that converts phosphatidylserine (PS) into phos￾phatidylethanolamine (PE). Eventually, PE is methylated to form phos￾phatidylcholine (PC) by the phospholipid methyl transferase enzymes I and II (PMT I and II). (4) The accumulation of PC on the exterior sur￾face of the plasma membrane causes an increase in membrane fluidity and facilitates the formation of Ca2+ channels. The resulting influx of Ca2+ activates phospholipase A2, which promotes the breakdown of PC into lysophosphatidylcholine (lyso PC) and arachidonic acid. (5) Arachidonic acid is converted into potent mediators: the leuko￾trienes and prostaglandin D2. (6) FcRI crosslinkage also activates the membrane adenylate cyclase, leading to a transient increase of cAMP within 15 s. A later drop in cAMP levels is mediated by protein kinase and is required for degranulation to proceed. (7) cAMP-dependent pro￾tein kinases are thought to phosphorylate the granule-membrane pro￾teins, thereby changing the permeability of the granules to water and Ca2+ . The consequent swelling of the granules facilitates fusion with the plasma membrane and release of the mediators

Hypersensitive Reactions CHAPTER 16 369 FIGURE 16-7 Kinetics of major bio- chemical events that follow crosslinkage of bound igE on cultured human ba ex IgE Curves are shown for phospholipid methylation(solid blue). CAMP produc. (solid black ) Ca + influx(dashed blue), and histamine release(dashed black). In control experiments with e anti-IgE Fab fragments, no significant ine release changes were observed. /Adapted from T. Ishizaka et al., 1985, Int. Arch. Allergy 2宝 Appl ImmunoL. 77: 137) Anti-IgE Fab 2 Time. min as an amplifier and effector of an antigen-antibody interac- receptors on various target cells. Three types of histamine re- tion. When generated in response to parasitic infection, these ceptors-designated H1, H2, and Hy-have been identified; mediators initiate beneficial defense processes, including these receptors have different tissue distributions and medi vasodilation and increased vascular permeability, which ate different effects when they bind histamine brings an influx of plasma and inflammatory cells to attack Most of the biologic effects of histamine in allergic reac the pathogen. On the other hand, mediator release induced tions are mediated by the binding of histamine to H, recep by inappropriate antigens, such as allergens, results in unnec- tors. This binding induces contraction of intestinal and bron essary increases in vascular permeability and inflammation chial smooth muscles, increased permeability of venules, and hose detrimental effects far outweigh any beneficial effect. increased mucus secretion by goblet cells. Interaction of his The mediators can be classified as either primary or sec- tamine with H2 receptors increases vasopermeability and ondary (Table 16-3). The primary mediators are produced dilation and stimulates exocrine glands. Binding of hista before degranulation and are stored in the granules. The mine to H2 receptors on mast cells and basophils suppresses most significant primary mediators are histamine, proteases, degranulation; thus, histamine exerts negative feedback on eosinophil chemotactic factor, neutrophil chemotactic fac- the release of mediators tor, and heparin. The secondary mediators either are synthe- sized after target-cell activation or are released by the break- LEUKOTRIENES AND PROSTAGLANDINS down of membrane phospholipids during the degranulation As secondary mediators, the leukotrienes and prostaglandins process. The secondary mediators include platelet-activating are not formed until the mast cell undergoes degranulation factor,leukotrienes, prostaglandins, bradykinins, and various and the enzymatic breakdown of phospholipids in the cytokines. The differing manifestations of type I hypersens- plasma membrane. An ensuing enzymatic cascade generates tivity in different species or different tissues partly reflect the prostaglandins and the leukotrienes(see Figure 16-6).It variations in the primary and secondary mediators present. therefore takes a longer time for the biological effects of these The main biological effects of several of these mediators are mediators to become apparent. Their effects are more pro- described briefly in the next sections nounced and longer lasting, however, than those of histamine leukotrienes mediate bronchoconstriction, increased vas- HISTAMIN cular permeability, and mucus production. Prostaglandin D2 Histamine, which is formed by decarboxylation of the amino causes bronchoconstriction. acid histidine, is a major component of mast-cell granules, The contraction of human bronchial and tracheal smooth accounting for about 10% of granule weight. Because it is muscles appears at first to be mediated by histamine, but tored-preformed-in the granules, its biological effects are within 30-60 s, further contraction is mediated by the leuko- observed within minutes of mast-cell activation. Once re- trienes and prostaglandins. Being active at nanomole levels, leased from mast cells, histamine initially binds to specific the leukotrienes are as much as 1000 times more potent

as an amplifier and effector of an antigen-antibody interac￾tion. When generated in response to parasitic infection, these mediators initiate beneficial defense processes, including vasodilation and increased vascular permeability, which brings an influx of plasma and inflammatory cells to attack the pathogen. On the other hand, mediator release induced by inappropriate antigens, such as allergens, results in unnec￾essary increases in vascular permeability and inflammation whose detrimental effects far outweigh any beneficial effect. The mediators can be classified as either primary or sec￾ondary (Table 16-3). The primary mediators are produced before degranulation and are stored in the granules. The most significant primary mediators are histamine, proteases, eosinophil chemotactic factor, neutrophil chemotactic fac￾tor, and heparin. The secondary mediators either are synthe￾sized after target-cell activation or are released by the break￾down of membrane phospholipids during the degranulation process. The secondary mediators include platelet-activating factor, leukotrienes, prostaglandins, bradykinins, and various cytokines. The differing manifestations of type I hypersensi￾tivity in different species or different tissues partly reflect variations in the primary and secondary mediators present. The main biological effects of several of these mediators are described briefly in the next sections. HISTAMINE Histamine, which is formed by decarboxylation of the amino acid histidine, is a major component of mast-cell granules, accounting for about 10% of granule weight. Because it is stored—preformed—in the granules, its biological effects are observed within minutes of mast-cell activation. Once re￾leased from mast cells, histamine initially binds to specific receptors on various target cells. Three types of histamine re￾ceptors—designated H1, H2, and H3—have been identified; these receptors have different tissue distributions and medi￾ate different effects when they bind histamine. Most of the biologic effects of histamine in allergic reac￾tions are mediated by the binding of histamine to H1 recep￾tors. This binding induces contraction of intestinal and bron￾chial smooth muscles, increased permeability of venules, and increased mucus secretion by goblet cells. Interaction of his￾tamine with H2 receptors increases vasopermeability and dilation and stimulates exocrine glands. Binding of hista￾mine to H2 receptors on mast cells and basophils suppresses degranulation; thus, histamine exerts negative feedback on the release of mediators. LEUKOTRIENES AND PROSTAGLANDINS As secondary mediators, the leukotrienes and prostaglandins are not formed until the mast cell undergoes degranulation and the enzymatic breakdown of phospholipids in the plasma membrane. An ensuing enzymatic cascade generates the prostaglandins and the leukotrienes (see Figure 16-6). It therefore takes a longer time for the biological effects of these mediators to become apparent. Their effects are more pro￾nounced and longer lasting, however, than those of histamine. The leukotrienes mediate bronchoconstriction, increased vas￾cular permeability, and mucus production. Prostaglandin D2 causes bronchoconstriction. The contraction of human bronchial and tracheal smooth muscles appears at first to be mediated by histamine, but, within 30–60 s, further contraction is mediated by the leuko￾trienes and prostaglandins. Being active at nanomole levels, the leukotrienes are as much as 1000 times more potent as Hypersensitive Reactions CHAPTER 16 369 45Ca uptake, cpm × 10–3/106 cells ( ) Histamine release, % ( ) 8 6 4 2 50 30 10 Methylation cAMP Ca2+ uptake Anti-IgE Fab Histamine release 1 2 3 5 8 10 Time, min [3H] Methyl incorporation, cpm × 10–3/106 cells ( ) cAMP, pmol/106 cells ( ) 6 4 2 6 5 4 3 2 FIGURE 16-7 Kinetics of major bio￾chemical events that follow crosslinkage of bound IgE on cultured human ba￾sophils with F(ab )2 fragments of anti￾IgE. Curves are shown for phospholipid methylation (solid blue), cAMP produc￾tion (solid black), Ca2+ influx (dashed blue), and histamine release (dashed black). In control experiments with anti–IgE Fab fragments, no significant changes were observed. [Adapted from T. Ishizaka et al., 1985, Int. Arch. Allergy Appl. Immunol. 77:137.]

370 paRI I Immune Effector mechanisms TABLE 16-3 Principal mediators involved in type I hypersensitivity Mediat Effects PRIMARY Histamine, heparin Increased vascular permeability; smooth-muscle contraction serotonin Increased vascular permeability; smooth-muscle contraction Eosinophil chemotactic factor(ECF-A) eosinophil chemotaxis Neutrophil chemotactic factor (NCF. eutrophil chemotaxis Proteases Bronchial mucus secretion; degradation of blood-vessel basement membrane generation of complement split products SECONDARY Platelet-activating factor Platelet aggregation and degranulation; contraction of pulmonary smooth muscles Leukotrienes(slow reactive substance of anaphylaxis, SRS-A Increased vascular permeability: contraction of pulmonary smooth muscles Prostaglandins Vasodilation contraction smooth muscles; platelet aggregation B Increased vascular perme ooth-muscle contraction IL-1 and TNF-c Systemic anaphylaxis; increased expression of CAMs on venular endothelial cells IL-2, IL-3, IL-4, IL-5, IL-6, TGF-B, and GM-CSF Various effects(see Table 12-1) bronchoconstrictors than histamine is, and they are also reaction. This was the response observed by portier and more potent stimulators of vascular permeability and mucus Richet in dogs after antigenic challenge. Systemic anaphy secretion. In humans, the leukotrienes are thought to con laxis can be induced in a variety of experimental animals and tribute to the prolonged bronchospasm and buildup of mu- is seen occasionally in humans. Each species exhibits charac Is seen in asthmatics teristic symptoms, which reflect differences in the distribu tion of mast cells and in the biologically active contents of CYTOKINES their granules. The animal model of choice for studying sys- Adding to the complexity of the type I reaction is the variety temic anaphylaxis has been the guinea pig Anaphylaxis can of cytokines released from mast cells and eosinophils. Some be induced in guinea pigs with ease, and its symptoms closely of these may contribute to the clinical manifestations of type I hypersensitivity. Human mast cells secrete IL-4, IL-5, IL-6, Active sensitization in guinea pigs is induced by a single and TNF-a These cytokines alter the local microenviron- injection of a foreign protein such as egg albumin. After an ment, eventually leading to the recruitment of inflammatory incubation period of about 2 weeks. the animal is usually cells such as neutrophils and eosi inophils. IL-4 increases ige challenged with an intravenous injection of the same pro production by B cells. IL-5 is especially important in the tein. Within 1 min, the animal becomes restless, its respira- recruitment and activation of eosinophils. The high concen- tion becomes labored, and its blood pressure drops.As the trations of TNF-a secreted by mast cells may contribute to smooth muscles of the gastrointestinal tract and bladder shock in systemic anaphylaxis. (This effect may parallel the contract, the guinea pig defecates and urinates. Finally bron chiole constriction results in death by asphyxiation within role of TNF-a in bacterial septic shock and toxic-shock syn- 2-4 min of the injection. These events all stem from the sys- drome described in Chapter 12. temic vasodilation and smooth-muscle contraction brought on by mediators released in the course of the reaction. Post Type I Reactions Can Be Systemic mortem examination reveals that massive edema, shock, and or localized bronchiole constriction are the major of death The clinical manifestations of type I reactions can range from Systemic anaphylaxis in humans is characterized by a sim- ilar sequence of events. a wide range of antigens have been life-threatening conditions, such as systemic anaphylaxis and shown to trigger this reaction in susceptible humans, includ asthma, to hay fever and eczema, which are merely annoying. ing the venom from bee, wasp, hornet, and ant stings; drugs, such as penicillin, insulin, and antitoxins; and seafood and SYSTEMIC ANAPHYLAXIS If not treated quickly, these re be fatal. Epi- Systemic anaphylaxis is a shock-like and often fatal state nephrine is the drug of choice for systemic anaphylactic reac- hose onset occurs within minutes of a type I hypersensitive tions. Epinephrine counteracts the effects of mediators such

bronchoconstrictors than histamine is, and they are also more potent stimulators of vascular permeability and mucus secretion. In humans, the leukotrienes are thought to con￾tribute to the prolonged bronchospasm and buildup of mu￾cus seen in asthmatics. CYTOKINES Adding to the complexity of the type I reaction is the variety of cytokines released from mast cells and eosinophils. Some of these may contribute to the clinical manifestations of type I hypersensitivity. Human mast cells secrete IL-4, IL-5, IL-6, and TNF- These cytokines alter the local microenviron￾ment, eventually leading to the recruitment of inflammatory cells such as neutrophils and eosinophils. IL-4 increases IgE production by B cells. IL-5 is especially important in the recruitment and activation of eosinophils. The high concen￾trations of TNF- secreted by mast cells may contribute to shock in systemic anaphylaxis. (This effect may parallel the role of TNF- in bacterial septic shock and toxic-shock syn￾drome described in Chapter 12.) Type I Reactions Can Be Systemic or Localized The clinical manifestations of type I reactions can range from life-threatening conditions, such as systemic anaphylaxis and asthma, to hay fever and eczema, which are merely annoying. SYSTEMIC ANAPHYLAXIS Systemic anaphylaxis is a shock-like and often fatal state whose onset occurs within minutes of a type I hypersensitive reaction. This was the response observed by Portier and Richet in dogs after antigenic challenge. Systemic anaphy￾laxis can be induced in a variety of experimental animals and is seen occasionally in humans. Each species exhibits charac￾teristic symptoms, which reflect differences in the distribu￾tion of mast cells and in the biologically active contents of their granules. The animal model of choice for studying sys￾temic anaphylaxis has been the guinea pig. Anaphylaxis can be induced in guinea pigs with ease, and its symptoms closely parallel those observed in humans. Active sensitization in guinea pigs is induced by a single injection of a foreign protein such as egg albumin. After an incubation period of about 2 weeks, the animal is usually challenged with an intravenous injection of the same pro￾tein. Within 1 min, the animal becomes restless, its respira￾tion becomes labored, and its blood pressure drops. As the smooth muscles of the gastrointestinal tract and bladder contract, the guinea pig defecates and urinates. Finally bron￾chiole constriction results in death by asphyxiation within 2–4 min of the injection. These events all stem from the sys￾temic vasodilation and smooth-muscle contraction brought on by mediators released in the course of the reaction. Post￾mortem examination reveals that massive edema, shock, and bronchiole constriction are the major causes of death. Systemic anaphylaxis in humans is characterized by a sim￾ilar sequence of events. A wide range of antigens have been shown to trigger this reaction in susceptible humans, includ￾ing the venom from bee, wasp, hornet, and ant stings; drugs, such as penicillin, insulin, and antitoxins; and seafood and nuts. If not treated quickly, these reactions can be fatal. Epi￾nephrine is the drug of choice for systemic anaphylactic reac￾tions. Epinephrine counteracts the effects of mediators such 370 PART III Immune Effector Mechanisms TABLE 16-3 Principal mediators involved in type I hypersensitivity Mediator Effects PRIMARY Histamine, heparin Increased vascular permeability; smooth-muscle contraction Serotonin Increased vascular permeability; smooth-muscle contraction Eosinophil chemotactic factor (ECF-A) Eosinophil chemotaxis Neutrophil chemotactic factor (NCF-A) Neutrophil chemotaxis Proteases Bronchial mucus secretion; degradation of blood-vessel basement membrane; generation of complement split products SECONDARY Platelet-activating factor Platelet aggregation and degranulation; contraction of pulmonary smooth muscles Leukotrienes (slow reactive substance of anaphylaxis, SRS-A) Increased vascular permeability; contraction of pulmonary smooth muscles Prostaglandins Vasodilation; contraction of pulmonary smooth muscles; platelet aggregation Bradykinin Increased vascular permeability; smooth-muscle contraction Cytokines IL-1 and TNF- Systemic anaphylaxis; increased expression of CAMs on venular endothelial cells IL-2, IL-3, IL-4, IL-5, IL-6, TGF-, and GM-CSF Various effects (see Table 12-1)