White blood cell formed in lymphoid tissue as in the lymph nodes, thymus, spleen & tonsils. Constitutes about 22-28% of all leukocytes in normal human's blood. Lymphocytosis is seen in infectious mononucleosis, viral hepatitis, cytomegalovirus infection, other viral infections, pertussis, toxoplasmosis, brucellosis, TB, syphilis, lymphocytic leukemias, and lead, carbon disulfide, tetrachloroethane, and arsenical poisonings. A mature lymphocyte count more than 7,000/uL is an individual over 50 years of age is highly suggestive of chronic lymphocytic leukemia (CLL). Drugs increasing the lymphocyte count include aminosalicyclic acid, griseofulvin, haloperidol, levodopa, niacinamide, phenytoin, and mephenytoin. Lymphopenia is characteristic of AIDS. It is also seen in acute infections, Hodgkin's disease, systemic lupus, renal failure, carcinomatosis, and with administration of corticosteroids, lithium, mechlorethamine, methysergide, niacin, and ionizing irradiation. Of all hematopoietic cells lymphocytes are the most sensitive to whole-body irradiation, and their count is the first to fall in radiation sickness