T lymphocytes recognize antigen through specific interactions with MHC molecules. The diverse, antigen-specific T-cell receptors entrusted with this task are structurally and biosynthetically related to the F(ab) portion of immunoglobulins. To generate a large repertoire of antigen-specific T-cell receptors, T cells rearrange the genes encoding the T-cell receptor subunits by mechanisms of somatic recombination essentially identical to those used by B cells to rearrange immunoglobulin genes. And the development of T cells, like that of B cells, is strictly dependent on successful completion of these somatic gene rearrangements to yield a functional T-cell receptor. In this section, we describe the receptor subunits that mediate antigen-specific recognition, how they pair up with membrane glycoproteins essential for signal transduction, and how these complexes recognize MHC-peptide combinations.

As pointed out in the preceding section, an individual’s T cells recognize peptide antigens only when they are bound to the polymorphic MHC molecules present in that individual. In the course of T-cell development, T cells must “learn” the identity of these “self” MHC molecules and receive instructions about which MHC-peptide combinations to ignore, so as to avoid potentially catastrophic reactions of newly generated T cells with the individual’s own tissues (i.e., autoimmunity).