Lymphopoiesis - The Process of Lymphopoiesis - Lymphopoiesis For T Cells

Lymphopoiesis For T Cells

T cells are formed in bone marrow then migrate to the cortex of the thymus to undergo maturation in an antigen-free environment for about one week where a mere 2-4% of the T cells succeed. The remaining 96-98% of T cells die by apoptosis and are 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 (Tregs)

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 (13–15μm). This large lymphocyte (known in this context as a lymphoblast) then divides several times to produce an expanded population of medium (9–12μm) and small lymphocytes (5–8μm) 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:

1. Prolymphocyte
2. Large lymphocyte
3. 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)