Definition: A white blood cell formed in bone marrow and distributed throughout the body in lymphatic tissue (lymph nodes, spleen, thymus, tonsils, Peyer patches), where it undergoes proliferation. In normal adults, lymphocytes make up 22–28% of the total number of leukocytes in the circulating blood. Lymphocytes generally are small (7–8 mcm), although larger forms (10–20 mcm) occur frequently. With Wright or similar stain, the nucleus is colored a deep purple-blue and is composed of dense aggregates of chromatin within a sharply defined nuclear membrane; the nucleus usually is round but may be slightly indented, and is eccentrically situated within a relatively small amount of light-blue cytoplasm that ordinarily contains no granules; especially in larger forms, the cytoplasm may be fairly abundant and include several fine granules of bright red-violet; unlike granules of the myeloid series of cells, those in lymphocytes do not yield a positive oxidase or perioxidase reaction. Lymphocytes are divided into two principal groups, T and B cells, based on their function and on characteristics of their surface molecules. Null cells, which include natural killer cells, represent a small percentage of the lymphocyte population.
[lympho- + G. kytos, call]
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