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Any of the mononuclear, nonphagocytic leukocytes, found in the blood, lymph, and lymphoid tissues, that are the body's immunologically competent cells and their precursors. They are divided on the basis of ontogeny and function into two classes, B and T lymphocytes, responsible for humoral and cellular immunity, respectively. Most are small lymphocytes, 7-10 mc in diameter with a round or slightly indented heterochromatic nucleus that almost fills the cell and a thin rim of basophilic cytoplasm that contains few granules. When activated by contact with antigen, small lymphocytes begin macromolecular synthesis, the cytoplasm enlarges until the cells are 10-30 mc in diameter, and the nucleus becomes less completely heterochromatic; they are then referred to as large lymphocytes of lymphoblasts. These cells then proliferate and differentiate into B and T memory cells and into the various effector cell types, B cells into plasma cells and T cells into helper, cytotoxic, and suppressor cells.


Taylor, E.J., Dorland's Illustrated Medical Dictionary 27th Edition, W.B. Saunders, Co. Publishing, Philadelphia, page 177. 1988.