15-20 T lymphocytes regulate the immune system and fight intracellular infections, viral infection and cancer

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• T cells respond to antigens using a T cell receptor that is similar in structure to antibodies.
• Antigen presenting cells present antigen to T cells inside a major histocompatibility complex protein.

T cells are the second arm of the immune system and are part of what is called cell-mediated immunity. Their roles are to regulate the immune system, help B lymphocytes in the production of antibodies and deal with threats to our body that involve our own cells (e.g., intracellular bacterial infections, viral infections and cancer). T cells recognize antigens in our bodies using receptors on their surface, sensibly called T cell receptors (TCR) as shown in Figure 15-20. These proteins are somewhat reminiscent of antibodies and are composed of two protein fragments, α and β. Each polypeptide chain can be broken into three domains. The carboxy-terminal domain serves as a hydrophobic anchor in the cellular membrane and also contains a disulfide bridge near the outside membrane surface that links the α and β polypeptides. The middle of each protein forms a constant region that shares homology across all TCRs and serves as scaffold for the variable domain projecting furthest into the surrounding environment. TCRs occur in association with CD3 (another protein on the surface of T cells) to form a TCR-CD3 complex. It is this complex that recognizes presented antigens and begins the T cell activation process.

The variable regions of these receptors react with antigens in a fashion that is reminiscent of antigen-antibody reactions. TCRs can respond to a large assortment of molecules and this diversity is achieved by a mechanism similar to that used to create antibody diversity as discussed previously. In contrast to antibodies, TCRs respond to antigens only when it is associated with a membrane protein of another host cell such as antigen-presenting cells (APC). These APCs wrap fragments of foreign molecules inside special proteins, called Major Histocompatibility Complex (MHC) molecules and then display them on their surface for presentation to T cells. In order to understand the mechanism of T cell activation, it is useful to first learn about these important protein complexes that every animal cell has.

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