T lymphocytes

the cells primarily responsible for cell-mediated immunity; they originate from lymphoid stem cells that migrate from the bone marrow to the thymus and differentiate under the influence of the thymic hormones thymopoietin and thymosin. They are characterized by specific surface antigens: the pan-T antigens Thy-1 (murine) and CD3 (human) are found on all mature T lymphocytes; other markers characterize T cell subsets. T cell antigen receptors are triggered by antigen only when associated with self MHC antigens, e.g., by antigens processed and presented by macrophages, viral antigens on the surface of host cells, and tumor neoantigens. When activated by antigen, T lymphocytes proliferate and differentiate into T memory cells and the various types of regulatory and effector T cells; see cytotoxic T l's and helper, suppressor, contrasuppressor, and T_DTH cells, under cell. Called also T cells and thymus-dependent l's.