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Before discussing how helper T cells function to activate macrophages, cytotoxic T cells, or B cells, we need to consider the crucial role of MHC proteins in T cell responses. Once bound to its target cell, a cytotoxic T cell can employ at least two strategies to kill the target, both of which operate by inducing the target cell to kill itself by undergoing apoptosis . Perforin is stored in secretory vesicles of the cytotoxic T cell and is released by local exocytosis at the point of contact with the target cell. The secretory vesicles also contain serine proteases, which are thought to enter the target cell cytosol through the perforin channels. One of the proteases, called granzyme B, cleaves, and thereby activates, one or more members of the caspase family of proteases that mediate apoptosis.

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The β2-microglobulin does not span the membrane and is encoded by a gene that does not lie in the MHC gene cluster. The α chain is folded into three extracellular globular domains (α1, α2, α3), and the α3 domain and the β2-microglobulin, which are closest to the membrane, are both similar to an Ig domain. The two N-terminal domains of the α chain, which are farthest from the membrane, contain the polymorphic amino acids that are recognized by T cells in transplantation reactions.

The most potent of these are dendritic cells (Figure 24-43), whose only known function is to present foreign antigens to T cells. Immature dendritic cells are located in tissues throughout the body, including the skin, gut, and respiratory tract. When they encounter invading microbes at these sites, they endocytose the pathogens or their products and carry them via the lymph to local lymph nodes or gut-associated lymphoid organs. The encounter with a pathogen induces the dendritic cell to mature from an antigen-capturing cell to an antigen-presenting cell that can activate T cells (see Figure 24-5). First, however, we need to consider how cytotoxic and helper T cells function and the special ways in which they recognize foreign antigen.

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A small minority of T cells, instead of making α and β chains, make a different but related type of receptor heterodimer, composed of γ and δ chains. These cells arise early in development and are found mainly in epithelia . Their functions are uncertain, and we shall not discuss them further. The pools of gene segments that encode the α and β chains are located on different chromosomes.

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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. 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. Indeed, the same VJ recombinase is used, including the RAG proteins discussed earlier.

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These caspases then activate other caspases, producing a proteolytic cascade that helps kill the cell (Figure 24-46A). Mice in which the perforin gene is inactivated cannot generate microbe-specific cytotoxic T cells and show increased susceptibility to certain viral and intracellular bacterial infections. They provide this protection by killing the infected cell before the microbes can proliferate and escape from the infected cell to infect neighboring cells. Before discussing the role of MHC proteins in presenting antigen to T cells, we consider the functions of the two major classes of T cells. Three types of proteins on the surface of an antigen-presenting cell involved in activating a T cell. The invariant polypeptide chains that are stably associated with the T cell receptor are not shown. There are three main types of antigen-presenting cells in peripheral lymphoid organs that can activate T cells—dendritic cells, macrophages, and B cells.

These domains bind a peptide and present it to cytotoxic T cells. Class I and class II MHC proteins have very similar overall structures. They are both transmembrane heterodimers with extracellular N-terminal domains that bind antigen for presentation to T cells.

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