Lymphocyte

a white blood cell formed in lymphatic tissue throughout the body (e.g., lymph nodes, spleen, thymus, tonsils, Peyer patches, and sometimes in bone marrow) and in normal adults making up approximately 22–28% of the total number of leukocytes in the circulating blood

Description
Lymphocytes are generally small (7–8 &micro;m), but larger forms are frequent (10–20 &micro;m); with Wright (or a similar) stain, the nucleus is deeply colored (purple-blue), and is composed of dense aggregates of chromatin within a sharply defined nuclear membrane; the nucleus is usually 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 bright red-violet fine granules; in contrast to granules of the myeloid series of cells, those in lymphocytes do not yield a positive oxidase or peroxidase reaction. Lymphocytes are divided into 2 principal groups, termed T and B cells, based on their surface molecules as well as function. Natural killer cells, which are large granular lymphocytes, represent a small percentage of the lymphocyte population.