ety>[ From Thymus, the site of maturation of T-cells. ] A type of white blood cell that circulates in the blood and lymph, and provides cell-mediated immunity for the organism, protecting against infecting cells or the body's own malignant cells; also called T lymphocyte. There are several types of T cells. They develop, as do B cells, from progenitor cells in the bone marrow, but are distinguished from B-cells (B-lymphocytes) by their site of differentiation; T-cells mature in the thymus and B-cells in the bone marrow (in birds in the Bursa of Fabricius). They also have different antigen receptors from those of B-cells. T-cells differentiate into cells that can directly kill infecting cells (cell-mediated immunity, cytotoxity) or activate other cells of the immune system (helper T cells), whereas B-cells differentiate on activation into antibody-secreting plasma cells. Helper T cells interact with B-cells by secreting lymphokines that stimulate the B cell which have detected a foreign antigen to enter the cell cycle and develop, by repeated mitosis, into a clone of cells with identical receptors, and then to secrete antibodies to that specific antigen. [ PJC ]