Mononuclear phagocytes originate from stem cells in the bone marrow which differentiate from monoblasts into promonocytes, then into circulating blood monocytes. Subsequently the monocytes can develop into macrophages and reside in a variety of tissues. Mononuclear phagocytes have cell surface receptors for a variety of substances (e.g., IgG, complement components, fibronectin, and sugars) and are capable of secreting a number of mediators (enzymes, complement components, coagulation components, and monokines). The tissue macrophages adapt to their environment and express unique differentiated functions that are related to various anatomic sites and organs (e.g., Kupffer cells, pulmonary alveolar macrophages, osteoclasts, microglia). Macrophages have the capacity to become "activated" by both specific and nonspecific immunologic stimuli and the "activated" macrophage has enhanced functional capabilities (e.g., tumoricidal, microbicidal, phagocytosis, secretion of mediators). Abnormal monocyte/macrophage function may be acquired or may be due to genetic or developmental disorders. Because of their central role in host defense (in inflammatory responses, in antigen presentation, and in immunoregulatory networks), monocyte/macrophage dysfunction may result in one or more pathophysiologic consequences: defects in monocyte maturation, deficiencies in the clearance of physiologic substrates in lysosomal diseases (e.g., Gaucher's disease, mucopolysaccharidoses, osteopetrosis, metachromatic leukodystrophy), decreased synthesis and secretion of mediators (complement component deficiencies), defects in microbicidal activity (chronic granulomatous disease) and defects which are acquired following infection and during chemotherapy (e.g., acquired immune deficiency syndrome).