The maturation of most T cells and the development of their specific receptors are directed by the thymus and its hormones. This contrasts with B cells which develop in the bone marrow. Other T cells reach full maturity in the gastrointestinal tract.
In addition to the antigen specific T cell receptor, all mature T lymphocytes express coreceptors called CD3. CD3 molecuels surround the T cell receptor and assist in binding. T cells also express either a CD4 or a CD8 coreceptor. CD4 is an accessory receptor protein mostly found on T helper cells that helps the T cell receptor bind to MHC class II molecules. CD8 is mostly found on cytotoxic T cells that helps bind MHC class I molecuels.