8536d_ch03_057-075 8/6/02 10:28 AM Page 58 mac79 Mac 79: 45_Bw Glasby et al. Immunology 5e 58 PART II Generation of B-Cell and T-Cell Respons TABLE 3-1 experimental antigens used in on Molecular weight of some comm to a cow but is strongly immunogenic when injected into a abbit. Moreover, BSA would be expected to exhibit greater immunology immunogenicity in a chicken than in a goat, which is more closely related to bovines. There are some exceptions to Antigen Approximate molecular mass(Da) rule. Some macromolecules (e.g, collagen and cytochrome c) have been highly conserved throughout evolution and Bovine gamma globul 150,000 therefore display very little immunogenicity across diverse specles nents(e. g Bovine serum albumin 69,000 corneal tissue and sperm) are effectively sequestered from the immune system, so that if these tissues are injected even 40,000 into the animal from which they originated, they will func- 15,000 tion as immunogens. MOLECULAR SIZE Keyhole limpet hemocyanin >2,000,000 (KLH) There is a correlation between the size of a macromolecule Ovalbumin(OVA) 44.000 and its immunogenicity. The most active immunogens tend to have a molecular mass of 100,000 daltons( Da). Generally, 17,000 substances with a molecular mass less than 5000-10000 Da are poor immunogens, although a few substances with a Tetanus toxoid () 150,000 molecular mass less than 1000 Da have proven to be im- CHEMICAL COMPOSITION AND HETEROGENEITY Immunogenicity is not an intrinsic property of an antige Size and foreignness are not, by themselves, sufficient to out rather depends on a number of properties of the partic lar biological system that the antigen encounters. The next make a molecule immunogenic; other properties are needed wo sections describe the properties that most immunogens as well. For example, synthetic homopolymers(polymers are and the contribution that the biological system makes composed of a single amino acid or sugar)tend to lack im- to the expression of immunogenicity. munogenicity regardless of their size. Studies have shown are usually more immunogenic than homopolymers f i hat copolymers composed of different amino acids or The Nature of the Immunogen Contributes to Immunogenicity onstituents. These studies show that chemical complexity contributes to immunogenicity. In this regard it is notable mmunogenicity is determined, in part, by four properties of that all four levels of protein organization--primary, sec the immunogen: its foreignness, molecular size, chemical ondary, tertiary, and quaternary-contribute to the struc- composition and complexity, and ability to be processed and tural complexity of a protein and hence affect its immun presented with an MHC molecule on the surface of an anti- genicity(Figure 3-1). gen-presenting cell or altered self-cell. LIPIDS AS ANTIGENS FOREIGNNESS Appropriately presented lipoidal antigens can induce B-and In order to elicit an immune response, a molecule must be T-cell responses. For the stimulation of B-cell responses, recognized as nonself by the biological system. The capacity lipids are used as haptens and attached to suitable carrier to recognize nonself is accompanied by tolerance of self, a molecules such as the proteins keyhole limpet hemocyanin pecific unresponsiveness to self antigens. Much of the ability (KLH)or bovine serum albumin(BSA). By immunizing with to tolerate self antigens arises during lymphocyte develop- these lipid-protein conjugates it is possible to obtain anti ment,during which immature lymphocytes are exposed to bodies that are highly specific for the target lipids. Using this self-components Antigens that have not been exposed to im- approach, antibodies have been raised against a wide variety mature lymphocytes during this critical period may be later of lipid molecules including steroids, complex fatty-acid de- recognized as nonself, or foreign, by the immune system. rivatives, and fat-soluble vitamins such as vitamin E Such When an antigen is introduced into an organism, the degree antibodies are of considerable practical importance since of its immunogenicity depends on the degree of its foreign- many clinical assays for the presence and amounts of med ess. Generally, the greater the phylogenetic distance be- ically important lipids are antibody-based. For example,a tween two species, the greater the structural (and therefore determination of the levels of a complex group of lipids For example, the common experimental antigen bovine tients. Prednisone, an immunosuppressive Stcr,0"? the antigenic)disparity between them. known as leukotrienes can be useful in evaluating asthma serum albumin(BSA) is not immunogenic when injected given as part of the effort to prevent the rejection of a transImmunogenicity is not an intrinsic property of an antigen but rather depends on a number of properties of the particu￾lar biological system that the antigen encounters. The next two sections describe the properties that most immunogens share and the contribution that the biological system makes to the expression of immunogenicity. The Nature of the Immunogen Contributes to Immunogenicity Immunogenicity is determined, in part, by four properties of the immunogen: its foreignness, molecular size, chemical composition and complexity, and ability to be processed and presented with an MHC molecule on the surface of an anti￾gen-presenting cell or altered self-cell. FOREIGNNESS In order to elicit an immune response, a molecule must be recognized as nonself by the biological system. The capacity to recognize nonself is accompanied by tolerance of self, a specific unresponsiveness to self antigens. Much of the ability to tolerate self antigens arises during lymphocyte develop￾ment, during which immature lymphocytes are exposed to self-components. Antigens that have not been exposed to im￾mature lymphocytes during this critical period may be later recognized as nonself, or foreign, by the immune system. When an antigen is introduced into an organism, the degree of its immunogenicity depends on the degree of its foreign￾ness. Generally, the greater the phylogenetic distance be￾tween two species, the greater the structural (and therefore the antigenic) disparity between them. For example, the common experimental antigen bovine serum albumin (BSA) is not immunogenic when injected into a cow but is strongly immunogenic when injected into a rabbit. Moreover, BSA would be expected to exhibit greater immunogenicity in a chicken than in a goat, which is more closely related to bovines. There are some exceptions to this rule. Some macromolecules (e.g., collagen and cytochrome c) have been highly conserved throughout evolution and therefore display very little immunogenicity across diverse species lines. Conversely, some self-components (e.g., corneal tissue and sperm) are effectively sequestered from the immune system, so that if these tissues are injected even into the animal from which they originated, they will func￾tion as immunogens. MOLECULAR SIZE There is a correlation between the size of a macromolecule and its immunogenicity. The most active immunogens tend to have a molecular mass of 100,000 daltons (Da). Generally, substances with a molecular mass less than 5000–10,000 Da are poor immunogens, although a few substances with a molecular mass less than 1000 Da have proven to be im￾munogenic. CHEMICAL COMPOSITION AND HETEROGENEITY Size and foreignness are not, by themselves, sufficient to make a molecule immunogenic; other properties are needed as well. For example, synthetic homopolymers (polymers composed of a single amino acid or sugar) tend to lack im￾munogenicity regardless of their size. Studies have shown that copolymers composed of different amino acids or sugars are usually more immunogenic than homopolymers of their constituents. These studies show that chemical complexity contributes to immunogenicity. In this regard it is notable that all four levels of protein organization—primary, sec￾ondary, tertiary, and quaternary—contribute to the struc￾tural complexity of a protein and hence affect its immuno￾genicity (Figure 3-1). LIPIDS AS ANTIGENS Appropriately presented lipoidal antigens can induce B- and T-cell responses. For the stimulation of B-cell responses, lipids are used as haptens and attached to suitable carrier molecules such as the proteins keyhole limpet hemocyanin (KLH) or bovine serum albumin (BSA). By immunizing with these lipid-protein conjugates it is possible to obtain anti￾bodies that are highly specific for the target lipids. Using this approach, antibodies have been raised against a wide variety of lipid molecules including steroids, complex fatty-acid de￾rivatives, and fat-soluble vitamins such as vitamin E. Such antibodies are of considerable practical importance since many clinical assays for the presence and amounts of med￾ically important lipids are antibody-based. For example, a determination of the levels of a complex group of lipids known as leukotrienes can be useful in evaluating asthma pa￾tients. Prednisone, an immunosuppressive steroid, is often given as part of the effort to prevent the rejection of a trans- 58 PART II Generation of B-Cell and T-Cell Responses TABLE 3-1 Molecular weight of some common experimental antigens used in immunology Antigen Approximate molecular mass (Da) Bovine gamma globulin 150,000 (BGG) Bovine serum albumin 69,000 (BSA) Flagellin (monomer) 40,000 Hen egg-white lysozyme 15,000 (HEL) Keyhole limpet hemocyanin 2,000,000 (KLH) Ovalbumin (OVA) 44,000 Sperm whale myoglobin 17,000 (SWM) Tetanus toxoid (TT) 150,000 8536d_ch03_057-075 8/6/02 10:28 AM Page 58 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: