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.


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