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Janeway CA Jr, Travers P, Walport M, et al. Immunobiology: The Immune System in Health and Disease. 5th edition. New York: Garland Science; 2001.
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We have learned in Chapter 2 that the bodyis defended by innate immune responses, but these will only work to control pathogensthat have certain molecular patterns or that induce interferons and other secreted yetnon-specific defenses. Most crucially, they do not allow memory to form as they operateby receptors that are coded in the genome. Thus, innate immunity is good for preventingpathogens from growing freely in the body, but it does not lead to the most importantfeature of adaptive immunity, which is long-lasting memory of specific pathogen.
To recognize and fight the wide range of pathogens an individual will encounter, thelymphocytes of the adaptive immune system have evolved to recognize a great variety ofdifferent antigens from bacteria, viruses, and other disease-causing organisms. Theantigen-recognition molecules of B cells are the immunoglobulins, or Ig. Theseproteins are produced by B cells in a vast range of antigen specificities, each B cellproducing immunoglobulin of a single specificity (see Sections 1-8 to 1-10). Membrane-boundimmunoglobulin on the B-cell surface serves as the cell"s receptor for antigen, and isknown as the B-cell receptor (BCR). Immunoglobulin of the sameantigen specificity is secreted as antibodyby terminally differentiated B cells—the plasma cells. The secretion of antibodies,which bind pathogens or their toxic products in the extracellular spaces of the body, isthe main effector function of B cells in adaptive immunity.
Antibodies were the first molecules involved in specific immune recognition to becharacterized and are still the best understood. The antibody molecule has two separatefunctions: one is to bind specifically to molecules from the pathogen that elicited theimmune response; the other is to recruit other cells and molecules to destroy thepathogen once the antibody is bound to it. For example, binding by antibody neutralizesviruses and marks pathogens for destruction by phagocytes and complement, as describedin Section 1-14. These functions are structurallyseparated in the antibody molecule, one part of which specifically recognizes and bindsto the pathogen or antigen whereas the other engages different effector mechanisms. Theantigen-binding region varies extensively between antibody molecules and is thus knownas the variable region or Vregion. The variability of antibody molecules allows each antibody to bind adifferent specific antigen, and the total repertoire of antibodies made by a singleindividual is large enough to ensure that virtually any structure can be recognized. Theregion of the antibody molecule that engages the effector functions of the immune systemdoes not vary in the same way and is thus known as the constant region or C region. It comes in five main forms, which are each specialized foractivating different effector mechanisms. The membrane-bound B-cell receptor does nothave these effector functions, as the C region remains inserted in the membrane of the B cell. Its function is as a receptor that recognizes and binds antigen by the V regionsexposed on the surface of the cell, thus transmitting a signal that causes B-cellactivation leading to clonal expansion and specific antibody production.
The antigen-recognition molecules of T cells are made solely as membrane-bound proteinsand only function to signal T cells for activation. These T-cell receptors (TCRs) are related toimmunoglobulins both in their protein structure—having both V and C regions—and in thegenetic mechanism that produces their great variability (see Section 1-10 and Chapter4). However, the T-cell receptor differs from the B-cell receptor in animportant way: it does not recognize and bind antigen directly, but instead recognizesshort peptide fragments of pathogen protein antigens, which are bound to MHC molecules on the surfaces of othercells.
The MHC molecules are glycoproteins encoded in the large cluster of genes known as themajor histocompatibility complex(MHC) (see Sections 1-16 and1-17). Their most striking structural featureis a cleft running across their outermost surface, in which a variety of peptides can bebound. As we shall discuss further in Chapter5, MHC molecules show great genetic variation in the population, and eachindividual carries up to 12 of the possible variants, which increases the range ofpathogen-derived peptides that can be bound. T-cell receptors recognize features both ofthe peptide antigen and of the MHC molecule to which it is bound. This introduces anextra dimension to antigen recognition by T cells, known as MHCrestriction, because any given T-cell receptor is specific not simply for aforeign peptide antigen, but for a unique combination of a peptide and a particular MHCmolecule. The ability of T-cell receptors to recognize MHC molecules, and theirselection during T-cell development for the ability to recognize the particular MHCmolecules expressed by an individual, are topics we shall return to in Chapters 5 and 7.
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In this chapter we focus on the structure and antigen-binding properties ofimmunoglobulins and T-cell receptors. Although B cells and T cells recognize foreignmolecules in two distinct fashions, the receptor molecules they use for this task arevery similar in structure. We will see how this basic structure can accommodate greatvariability in antigen specificity, and how it enables immunoglobulins and T-cellreceptors to carry out their functions as the antigen-recognition molecules of theadaptive immune response.