T-helper cells are so named because they produce and secrete protein molecules called cytokines that perform various functions within the immune system that direct and 'help' the immune response. Within the Helper T-cell group are two further subsets known as Th1 (or type 1) and Th2 (or type 2). Cytotoxic T-cells play a central role in the destruction of tumour cells and virally-infected cells, and are also thought to have an important role in transplant rejection.
Differentiation of T-cells into CD4+ and CD8+ cells begins in the thymus during T-cell development. However, cells only become fully differentiated mature cells in the peripheral lymphoid system, during an active immune response.
All T-cells originate in the marrow of long bones (such as the femur), and are derived from hematopoetic stem cells. Hematopoetic progenitors that derive from these stem cells travel to the thymus via the lymphatic system. Upon reaching the thymus they divide to generate T-cell precursors known as immature thymocytes (Schwarz B A, Bhandoola A. 2006). Approximately 98% of these precursor cells die in the thymus without becoming fully-differentiated T-cells, due to selection processes called positive and negative selection. The 2% of cells that survive selection eventually leave the thymus to become mature T-cells.
At the beginning of the selection process, all thy ...