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lymphocyte

A type of white blood cell (leukocyte) in the immune system. They are the main type of cell found in lymph and make up 18-42% of circulating leukocytes. The lymph contains waste products and cellular debris, together with bacteria and proteins. They arise from by mitosis of stem cells in the bone marrow. Some lymphocytes remain in the bone marrow and mature to become B cells. Other lymphocytes migrate to the thymus and mature to become T cells. Following maturation, they enter the circulation and secondary lymphoid organs where they survey for invading pathogens and tumor cells. Lymphocyte activation occurs when lymphocytes are triggered through antigen-specific receptors on their cell surface. This causes the cells to proliferate and differentiate into specialized effector lymphocytes. (Types) • natural killer cell (NK cell): Functions in cell-mediated, cytotoxic innate immunity. • T lymphocyte (T cell): Functions in cell-mediated, cytotoxic adaptive immunity. Matures in the thymus. • B lymphocyte (B cell): Functions in humoral, antibody-driven adaptive immunity. Matures in the bone marrow. (Lymphoid organs) • Primary lymphoid organs: E.g. bone marrow, thymus. Create lymphocytes. • Secondary lymphoid organs (peripheral lymphoid organs): E.g. lymph nodes (lymphocyte concentration is the highest), spleen, tonsils, mucosa-associated lymphoid tissue. Fight off germs and foreign substances.