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 leukotrienes, and several regulatory molecules. See also immunity.