phagocytosed by macrophages in the thymus. So many thymocytes (T cells) die during the maturation process because there is intensive screening to make sure each thymocyte has the ability to recognize self-peptide:self-MHC complex and for self tolerance. The apoptosed thymocyte dies and is quickly recycled. Upon maturity, there are several forms of thymocytes including T-helper (needed for activation of other cells such as B cells and macrophages), T-cytotoxic (which kill virally infected cells), T-memory (T cells that remember antigens previously encountered), and T-suppressor cells (which moderate the immune response of other leukocytes). Also called T-regulatory cells (Treg) When T cells become activated they undergo a further series of developments. A small, resting T lymphocyte rapidly undergoes blastogenic transformation into a large lymphocyte (13–15 μm). This large lymphocyte (known in this context as a lymphoblast) then divides several times to produce an expanded population of medium (9–12 μm) and small lymphocytes (5–8 μm) with the same antigenic specificity. Final activated and differentiated T lymphocytes are once again morphologically indistinguishable from a small, resting lymphocyte. Thus the following developmental states may be noticed in sequence in blood tests: Prolymphocyte Large lymphocyte Small lymphocyte Basic Map of T Cell lymphopoiesis This basic map of T Cell formation, in sequence, is simplified and is akin to textbook descriptions, and may not reflect latest research.(Medical Immunology, p. 119) In the thymus MLP ETP DN1 (B; Mφ) DN2 (DC; NK) DN3 (γδ) DN4 DP (TNK; CD4; CD8; Treg) In the Periphery (Th1; Th2) T cell development Unlike other lymphoid lineages, T cell development occurs almost exclusively in the thymus. T-lymphopoiesis does not occur automatically but requires signals generated from the thymic stromal cells. Several stages at which specific regulators and growth factors are required for T cell development to proceed have been defined. Later in T cell development and its maturation these same regulatory factors again are used to influence T cell specialization. T cells are unique among the lymphocyte populations in their ability to further specialize as mature cells and become yet more mature. And T cells come in many flavors, for example: the conventional TcRαβ T cells; the so-called unconventional TcRγδ T cells; NKT cells; and T regulatory cells (Treg). Details regarding the developmental and life cycle of the unconventional T cells are less well-described compared to the conventional T cells. Stages of T cell maturation Stage One: Thymic Migration Multipotent lymphoid progenitors (MLP)