Relatively long lived phagocytic cell of mammalian tissues, derived from blood monocyte. Macrophages from different sites have distinctly different properties. Main types are peritoneal and alveolar macrophages, tissue macrophages (histiocytes), kupffer cells of the liver and osteoclasts. In response to foreign materials may become stimulated or activated. Macrophages play an important role in killing of some bacteria, protozoa and tumour cells, release substances that stimulate other cells of the immune system and are involved in antigen presentation. May further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Any of the large, mononuclear, highly phagocytic cells derived from monocytes, occurring in the walls of blood vessels (adventitial cells) and in loose connective tissue (histiocytes, phagocytic reticular cells). They are components of the reticuloendothelial system. Macrophages have their origin in the bone marrow, where they pass through the monoblast and promonocyte stages to the monocyte stage; the monocytes enter the blood and then the tissues, where they become macrophages. Macrophages are usually immobile but become actively mobile when stimulated by inflammation. Their functions include phagocytosis and pinocytosis, presentation of antigens to T and B lymphocytes, and secretion of a variety of products, including enzymes, several complement components and coagulation factors, some prostaglandins and leukotriene’s, and several regulatory molecules