RETICULOENDOTHELIAL SYSTEM

also called Macrophage System, or Mononuclear Phagocyte System, class of cells that occur in widely separated parts of the human body and that take up particular substances. These cells are part of the body's defense mechanisms. Reticuloendothelial cells are phagocytic; i.e., they can engulf and destroy bacteria, viruses, and other foreign substances. They also can ingest worn-out or abnormal body cells. Reticuloendothelial cells are derived from precursor cells in the bone marrow. These precursors develop into monocytes, phagocytic cells that are released into the bloodstream. Some monocytes remain in the general blood circulation, but most of them enter body tissues, where they develop into much larger phagocytic cells called macrophages. The great majority of macrophages remain as stationary cells within tissue, where they filter out and destroy foreign particles. Some of them break away, however, and wander through the circulation and within the intercellular spaces. Tissue macrophages differ in appearance and name because of their various locations. For example, reticulum cells line the sinuses of the lymph nodes, spleen, and bone narrow, while histiocytes are found in numerous subcutaneous tissues. Microglia occur in nervous tissue, alveolar macrophages in the air spaces of the lungs, and Kupffer cells in the liver. A single reticuloendothelial cell can phagocytize (engulf and destroy) microorganisms, cells, and even tiny fragments of foreign objects, such as bits of splinters and suture materials. Several mobile macrophages can surround larger foreign objects and coalesce into a single phagocytic cell. By their phagocytosis of foreign substances, macrophages form an important first line of defense against harmful particles that have reached the body's interior. The reticuloendothelial cells also participate in body defense through immune reactions, a complex set of events targeted at a specific foreign substance. The reaction is directed by white blood cells known as lymphocytes. One class of lymphocytes (B cells) can synthesize and secrete antibodies with the help of another class of lymphocytes (T cells). T cells are also capable of other immunological reactions not involving antibody production. Macrophages often appear to be a required factor in an immune reaction. It is believed that phagocytosis of the foreign substance by macrophages helps reveal the surface molecules (antigens) on the foreign substance that stimulate lymphocyte responses. The production of antibodies, in turn, greatly stimulates the phagocytic activity of the macrophages. Another important function of the reticuloendothelial cells is the destruction of worn-out or abnormal cells and tissues. The reticulum cells of the spleen in particular play a major role in the destruction of worn-out red blood cells and the recycling of hemoglobin, the oxygen-carrying pigment of the red blood cells. The reticulum cells break down old red blood cells and metabolize the hemoglobin to create hemosiderin, a pigment used to form new red blood cells. Disorders associated with the reticuloendothelial system include anemia caused by excessive destruction of red blood cells by reticulum cells. There are also malignant tumours related to reticuloendothelial cells that can be either localized or widespread throughout the body; reticulum-cell sarcoma is the most common such neoplasm and is usually located in the lymph nodes. Another condition, histiocytic medullary reticulosis, results from the diffuse proliferation of phagocytic cells. Niemann-Pick and Gauche's diseases are hereditary disorders characterized by abnormal products of lipid metabolism within the reticuloendothelial cells.