[The monocyte-macrophage system in the human].

Abstract The mononuclear-phagocyte system includes promonocytes and their precursors in the bone marrow, monocytes in circulation and macrophages in tissues. After maturation in the bone marrow newly formed monocytes enter the circulation and migrate into different tissues; the half-life of monocytes in the blood stream is approximately three days. Once in the tissue monocytes undergo transformation into tissue macrophages with functional properties that are characteristic for the environment in which they reside. Macrophages play a central role in the immune regulation by presenting antigen to T-lymphocytes; they participate in ingestion and killing of various invading microorganisms. In addition, macrophages synthesize a great number of substances involved in host defense and inflammation i.e. complement components, prostaglandins, IL-1, tumor necrosis factor-alpha and others. During infection, macrophages have the capacity to become "activated" by lymphokines and different bacterial products; "activated" macrophages have an increased tumoricidal and microbicidal activity against various microorganisms, synthesis and secretion of immune mediators is enhanced. Monocyte-macrophage dysfunctions have been described in various disorders: defective chemotaxis (corticosteroids, drug induced immunosuppression, AIDS, diabetes), defective phagocytosis (lupus erythematosus, deficiency of a membrane glycoprotein), microbicidal defect (chronic granulomatous disease), decreased cytotoxicity (Wiskott-Aldrich-Syndrome), deficiencies in the clearance of physiologic substrates in lysosomal diseases.