A variety of white blood cell produced in the LYMPHOID TISSUE and lymphatic glands (see LYMPHATICS; GLANDS) of the body. They contain a simple, rounded nucleus. They play an important part in the production of ANTIBODIES, and in the rejection of transplanted organs such as the heart (see TRANSPLANTATION). This they do in two different ways: what are known as B-lymphocytes produce antibodies, while T-lymphocytes attack and destroy antigens (see ANTIGEN) directly. The latter are known as T-lymphocytes because they are produced by the THYMUS GLAND. Their numbers are increased in TUBERCULOSIS and certain other diseases. Such an increase is known as LYMPHOCYTOSIS.