Cells of the Immune System Introductory article lan Todd,University of Nottingham,Nottingham,UK Artice Contents Avariety of cell types are important components of the immune system.They constitute some of the body's main defences against infection. Introduction The nctio of the ydnd the with vastly differing natures.ie.viruses.bacteria.fung .Basophils and Mast Cells snsoTh ocplhsnanddstnSeplhsntc&les,andarc fuales which tnety constiture he rough of th the bl en(mhisscusBand e rec tof the bodyhom mphoid tis The reeptors of a single lymphocyte are identical to each other and recognize a single antigen.However,milli ons c cell types cooperate with each other by means of direct ween cell surface molecules and via the whole population of ymphocytes in the human body.thus As summarized in Table 1 a number of criteria can be used to distinguish between different cell types of the relation become an enlarged.dividing lymphoblast.Some of the progeny differentiate into short-ved ectors of the attributes. ory cells which will be reactivated if there is subsequent exposure to the same antigen. B lymphocytes Lymphocytes The three major types of lymphocytes are called Bcells,T to the thymus for selection and maturation.The bone bind to exactly the same antigen (Fiqure 1).A B cell will only and lymphatic s stem.spending sor me time in the from T cells.The activated B cell unde ymph nodes divisions and some of thceslimgcelsdierentatemtg of restin distinguished:Bcells and the majority are small involved in antibody ynthesis OF LIFE 1Cells of the Immune System Ian Todd, University of Nottingham, Nottingham, UK A variety of cell types are important components of the immune system. They constitute some of the body’s main defences against infection. Introduction The main function of the immune system is to defend the body against a wide variety of pathogenic infectious agents with vastly differing natures, i.e. viruses, bacteria, fungi, protozoa and parasitic worms. The complexity of this task requires a sophisticated repertoire of mechanisms for the recognition of, and defence of the body against, these pathogens. This is achieved by an array of cells (and molecules which they secrete) which are dispersed through￾out the body and collectively constitute the immune system. Most of the major cell types of the immune system are derived from progenitors (stem cells) in the bone marrow. Many of the mature cells circulate in the bloodstream and are dispersed throughout tissues of the body, while some also congregate in specialized lymphoid tissues. Further￾more, in order to generate effective immunity, the various cell types cooperate with each other by means of direct interactions between cell surface molecules and via the molecules that they secrete. As summarized in Table 1, a number of criteria can be used to distinguish between different cell types of the immune system: these include developmental relation￾ships, phenotypic distinctions based on morphology or cell surface molecules (e.g. CD markers), and functional attributes. Lymphocytes The three major types of lymphocytes are called B cells, T cells and NK (natural killer) cells. They arise from lymphoid progenitors in the bone marrow: mammalian B cells fully develop here, whereas T cell precursors migrate to the thymus for selection and maturation. The bone marrow and thymus are thus known as primary lymphoid organs. Mature B and T cells circulate in the bloodstream and lymphatic system, spending some time in the secondary lymphoid tissues, i.e. the spleen, lymph nodes and mucosa-associated lymphoid tissues (MALT). Two morphological types of resting lymphocytes can be distinguished: B cells and the majority of T cells are small lymphocytes with a thin rim of cytoplasm surrounding the nucleus, whereas NK cells and some T cells are larger, have more cytoplasm and distinct cytoplasmic granules, and are known as large granular lymphocytes (LGLs). B and T lymphocytes are entirely responsible for adaptive or acquired immunity, i.e. the ability to recognize each pathogen in a specific way and to mount a faster and bigger response on repeated exposure to a particular pathogen (immunological memory). This is because B and T cells express surface receptors which specifically bind to materials that are foreign to the body (known as antigens). The receptors of a single lymphocyte are identical to each other and recognize a single antigen. However, millions of different antigen receptors are collectively expressed by the whole population of lymphocytes in the human body, thus conferring the ability to recognize a great many foreign antigens. A lymphocyte can be activated when it binds an antigen for which its receptors are specific, causing it to become an enlarged, dividing lymphoblast. Some of the progeny differentiate into short-lived effectors of the immune response while others become long-lasting mem￾ory cells which will be reactivated if there is subsequent exposure to the same antigen. B lymphocytes The B cells constitute 5–15% of human blood lympho￾cytes. The main function of a B cell is to secrete soluble recognition molecules called antibodies which specifically bind to an antigen recognized by that B cell. These antibodies (also known as immunoglobulins) are, in fact, the secreted form of a B cell’s surface antigen receptors and bind to exactly the same antigen (Figure1). A B cell will only produce antibodies when it has been activated by binding antigen; this activation process also usually requires help from T cells. The activated B cell undergoes multiple divisions and some of the resulting cells differentiate into antibody-secreting cells. These are known as plasma cells, and they possess copious rough endoplasmic reticulum involved in antibody synthesis. During activation, B cells can undergo two types of genetic changes that modify the nature of the antibodies Article Contents Introductory article . Introduction . Lymphocytes . Natural Killer Cells . Dendritic Cells . Follicular Dendritic Cells . Monocytes and Macrophages . Neutrophils . Eosinophils . Basophils and Mast Cells . Other Cells which Contribute to the Immune System ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 1