Any one of a group of white blood cells that are of crucial importance to the immune system. There are 2 principal types of lymphocyte: B- and T-lymphocytes. B-lymphocytes produce immunoglobulins or antibodies, which attach themselves to antigens (proteins) on the surfaces of bacteria. This starts a process leading to the destruction of the bacteria. The T-lymphocytes comprise 3 main groups of cells: killer (cytotoxic) cells, helper cells, and suppressor cells. The killer T-lymphocytes attach to abnormal cells (for example, tumour cells, cells that have been invaded by viruses, and those in transplanted tissue) and release chemicals called lymphokines, which help to destroy the abnormal cells. Helper T-cells enhance the activities of the killer T-cells and the B-cells, and also control other aspects of the immune response. Suppressor T-cells act to “switch off” the immune response. Some lymphocytes do not participate directly in immune responses, but serve as a memory bank for antigens that have been encountered.
A variety of white blood cell produced in the LYMPHOID TISSUE and lymphatic glands (see LYMPHATICS; GLAND) of the body. It contains a simple, rounded nucleus surrounded by protoplasm generally described as non-granular. Two varieties of lymphocyte are described, small and large, and together they form over 20 per cent of the white cells of the blood. 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 di?erent 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.
n. a variety of white blood cell (leucocyte), present also in the lymph nodes, spleen, thymus gland, gut wall, and bone marrow. With *Romanowsky stains, lymphocytes can be seen to have dense nuclei with clear pale-blue cytoplasm. Lymphocytes with scanty cytoplasm are small lymphocytes; those with abundant cytoplasm are large lymphocytes. There are normally 1.5–4.0 × 109 lymphocytes per litre of blood. They are involved in *immunity and can be subdivided into B lymphocytes (or B cells), which produce circulating antibodies, and T lymphocytes (or T cells), which are primarily responsible for cell-mediated immunity. T lymphocytes can differentiate into *helper T cells or *cytotoxic T cells. There is an increase in the number of lymphocytes in the blood and bone marrow in chronic lymphocytic *leukaemia. —lymphocytic adj.
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