T Cell Antigen Receptors and the lmmunoglobulin Supergene Family Review

The vertebrate immune response employs two categories of cells that react specifically with antigens: B lymphocytes react with soluble antigens and T lymphocytes react with antigens that are present on cell surfaces. The receptor molecules that mediate these specific immune responses have similar heterodimeric structures: light (L) and heavy(H) polypeptides for immunoglobulins serve as the antigen receptor for B cells, and u and p chains comprise the T cell antigen receptor. Light and heavy chains, as well as a and fl chains, have variable (V) regions that recognize antigen and constant (C) regions that are involved in membrane anchoring and possibly signal transmission. Different immunoglobulin heavy chain constant regions also mediate different effector functions such as complement fixation or the degranulation of mast cells. The population of T or B cells in individual animals exhibits an extremely diverse antigen-binding repertoire due to the diversity of antigen receptors. However, individual B cells express only a single type of functional receptor and the same is probably true of T cells. Antigenic challenge leads to clonal expansion of lymphocytes that have receptors specific for that particular antigen.