T-lymphocyte
T lymphocytes, also called T cell or T helpers, are one of two primary types of lymphocytes— B cells being the second type—that determine the specificity of immune response to antigens (foreign substances) in the body.T cells originate in the bone marrow and mature in the thymus. In the thymus, T cells multiply and differentiate into helper, regulatory, or cytotoxic T cells or become memory T cells. They are then sent to peripheral tissues or circulate in the blood or lymphatic system. Once stimulated by the appropriate antigen, helper T cells secrete chemical messengers called cytokines, which stimulate the differentiation of B cells into plasma cells (antibody-producing cells). Regulatory T cells act to control immune reactions, hence their name. Cytotoxic T cells, which are activated by various cytokines, bind to and kill infected cells and cancer cells.
Because the body contains millions of T and B cells, many of which carry unique receptors, it can respond to virtually any antigen.