1. Cell present in the blood and lymphatic tissue, derived from the stem cells from which blood cells arise, and are the main means of providing the body with immune capability. 2. A white blood cell that creates an immune response when activated by a foreign molecule (antigen). T lymphocytes or T cells develop in an organ called the thymus and are responsible for cell-mediated immunity, while B lymphocytes develop in the bone marrow and are responsible for the production of antibodies (immunoglobulins).