ABSTRACT
The immune system is conventionally divided into the humoral immune system, the phagocytic system, and the cell-mediated immune system.
The humoral system comprises circulating antibody, complement, and other circulating substances that serve to coat microorganisms and either destroy them or enable phagocytic cells to engulf them. The phagocytic system consists of circulating phagocytes (polymorphonuclear neutrophils) and tissue macrophages that ingest and kill infecting organisms. The cell-mediated immune system, in which thymus-derived lymphocytes (T lymphocytes) play a major role, recognizes new antigens, especially those within cells, and assists in destroying or containing intracellular pathogens.
