A protein molecule found attached to the plasma membrane of a T cell that recognizes foreign antigen displayed on the surface of host cells and activates an immune response by the T cell. A single T cell typically has about 30 000 such receptors. Each is similar, but not identical, to one arm of a Y-shaped antibody molecule (i.e. a Fab fragment). It consists of an α‎ chain and a β‎ chain (corresponding to the light and heavy chains of a Fab fragment), each of which has an outer variable region (designated V_α‎ or V_β‎) and an inner constant region (C_α‎ or C_β‎). The V_α‎ and V_β‎ regions together form the single antigen-binding site. Each chain has a short hinge region close to the membrane, where the two chains are linked by a disulphide bond, and a transmembrane region terminating in a short cytoplasmic tail inside the cell. The two major classes of T cells are characterized by the nature of the coreceptors that operate in conjunction with the T-cell receptor. The subset carrying CD4 coreceptors recognize MHC class II proteins, whereas cells carrying CD8 coreceptors recognize MHC class I. In either case, both the coreceptor and T-cell receptor bind to the complex of MHC molecule and processed antigen fragment on the surface of an antigen-presenting cell or infected cell, thereby triggering the intracellular signal pathway that activates the cell. The great diversity of potential antigens is matched by the formidable array of T-cell receptors, each with a slightly different chemical structure and hence different binding properties. This diversity is generated by somatic recombination in T-cell progenitors (see thymocyte), in a manner similar to that responsible for producing the vast number of different antibodies.