A T cell is a white blood cell that plays a central role in the adaptive immune system. T cells are mononuclear leukocytes and belong to the group of lymphocytes. T is the shortcut for thymus where these cells mature in. Immature T cells migrate to the thymus gland, where they mature and differentiate into various types of mature T cells and become active in the immune system in response to a hormone called thymosin and other factors. T cells that are potentially activated against the body's own tissues are normally killed or changed ("down-regulated") during this maturational process. About 98% of thymocytes die during the development processes in the thymus by failing either positive selection or negative selection, whereas the other 2% survive and leave the thymus to become mature immune competent T cells.