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 Vα or Vβ) and an inner constant region (Cα or Cβ). The Vα and Vβ 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.
Subjects: Biological Sciences.