T cells are primarily responsible for cell-mediated immunity. They originate in the bone marrow and mature in the thymus. In the thymus, T cells multiply and differentiate into helper, regulatory, cytotoxic T cells, or 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 cytokines, which stimulate the differentiation of B cells. Regulatory T cells act to control immune reactions and cytotoxic T cells, which are activated by various cytokines, bind to and kill infected cells and cancer cells.