A type of white blood cell found in lymph, blood, and other specialized tissue such as bone marrow and tonsils, constituting between 22 and 28 percent of all white blood cells in the blood of a normal adult human being. B- and T-lymphocytes are crucial components of the immune system. The B-lymphocytes are primarily responsible for antibody production. The T-lymphocytes are involved in the direct attack against living organisms. The helper T-lymphocyte, a subtype, is the main cell infected and destroyed by the AIDS virus.