Lymphocytes are primarily involved in the body's immune response mechanism. This involves complex phenomena which end in the development of humoral and cellular immunity.
Humoral immunity involves the production of antibodies (immunoglobulins) and is brought about by lymphocytes called B-cells. B-cells are bone marrow derived lymphocytes. After an antigen stimulates B-cells, they proliferate and transform into plasma cells, which produce specific antibodies.
Cellular immunity includes delayed hypersensitivity reactions, graft rejection, graft-versus-host reactions, defense against intracellular organisms, production of cytokines, and defense against neoplasms. Cellular immunity is mediated by lymphocytes called T-cells. T-cells are so named because they are dependent on the thymus for their production and development. The majority of T-cells are long-lived, with an average lifespan of 4.4 years, but it is known that some survive for as long as 20 years or more. T-cells are capable of leaving and re-entering the circulation many times during their long life. T and B cells cannot be differentiated when viewing blood films. They are identified through the use of immunologic cell markers.