![]() Thus, the affinity of the receptors remains low ( K a ~ 10 5-10 7 liters/ mole), even late in an immune response. The mechanism that does not operate in T cell receptor diversification is antigen-driven somatic hypermutation. Indeed, the same V(D)J recombinase is used, including the RAG proteins discussed earlier. ![]() With one exception, all the mechanisms used by B cells to generate antibody diversity are also used by T cells to generate T cell receptor diversity. Like antibody heavy-chain pools, the T cell receptor pools contain separate V, D, and J gene segments, which are brought together by site-specific recombination during T cell development in the thymus. The pools of gene segments that encode the α and β chains are located on different chromosomes. Each chain is about 280 amino acids long and has a large extracellular part that is folded into two Ig-like domains-one (more.) (A) Schematic drawing showing that the receptor is composed of an α and a β polypeptide chain. We begin by considering the nature of the cell-surface receptors that T cells use to recognize antigen.Ī T cell receptor heterodimer. Finally, we describe how T cells are selected during their development in the thymus to ensure that only cells with potentially useful receptors survive and mature. We discuss how they recognize foreign antigens on the surface of antigen-presenting cells and target cells and consider the crucial part played by MHC proteins in the recognition process. In this section, we describe these two classes of T cells and their respective functions. Effector helper T cells, by contrast, help stimulate the responses of other cells-mainly macrophages, B cells, and cytotoxic T cells. Effector cytotoxic T cells directly kill cells that are infected with a virus or some other intracellular pathogen. There are two main classes of T cells-cytotoxic T cells and helper T cells. Activated B cells, by contrast, secrete antibodies that can act far away. They interact directly with another cell in the body, which they either kill or signal in some way (we shall refer to such cells as target cells). The second difference is that, once activated, effector T cells act only at short range, either within a secondary lymphoid organ or after they have migrated into a site of infection. The peptide fragments are then carried to the surface of the presenting cell on special molecules called MHC proteins, which present the fragments to T cells. Whereas B cells recognize intact antigen, T cells recognize fragments of protein antigens that have been partly degraded inside the antigen-presenting cell. The T cells respond in this manner because the form of antigen they recognize is different from that recognized by B cells. First, T cells are activated by foreign antigen to proliferate and differentiate into effector cells only when the antigen is displayed on the surface of antigen-presenting cells in peripheral lymphoid organs. T cell responses differ from B cell responses in at least two crucial ways. Much of the rest of this chapter is concerned with how T cells accomplish this feat. Most importantly, unlike B cells, T cells can help eliminate pathogens that reside inside host cells. Indeed, most adaptive immune responses, including antibody responses, require helper T cells for their initiation. Like antibody responses, T cell responses are exquisitely antigen-specific, and they are at least as important as antibodies in defending vertebrates against infection. This is to distinguish them from antibody responses, which, of course, also depend on cells (B cells). The diverse responses of T cells are collectively called cell-mediated immune reactions.
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