The CD8 glycoprotein is expressed by thymocytes, mature T cells and natural killer (NK) cells and has been implicated in the recognition of monomorphic determinants on major histocompatibility complex (MHC) Class I antigens, and in signal transduction during the course of T-cell activation. The CD8 glycoprotein is comprised of two distinct polypeptide chains, alpha and beta, which have the capacity to form homodimeric (CD8 alpha/alpha) or heterodimeric (CD8 alpha/beta) cell surface complexes. The CD8 antigen acts as a coreceptor with the T-cell receptor on the T lymphocyte to recognize antigens displayed by an antigen presenting cell in the context of class I MHC molecules.