n. (Biol.) The complex of cells, cellular processes, and substances within and diffused throughout an organism which allow the organism to counteract or destroy noxious foreign substances introduced into the body, destroy infectious agents such as bacteria and viruses, destroy malignant cells, and remove cellular debris, thus protecting the organism against many of the potentially harmful external agents and internal events that could lead to sickness or death. The system has numerous interacting components, including circulating antibodies, antibody-producing cells, white blood cells and lymphokines, lymph tissue and lymph nodes, and stem cells which may differentiate into other types of cell, together with the thymus and spleen. The system is responsible for the phenomenon of immunity{ 3 }. See also immunoglobulin and antibody. [ PJC ]