A term that encompasses the monocyte–macrophage or mononuclear phagocytic system and also includes reticulum cells. It consists of a population of cells called macrophages, which are of bone marrow origin and are widely distributed throughout the body. Immature macrophages or monocytes make up about 5% of peripheral blood leukocytes. Mature macrophages are found in connective tissue (histiocytes), lining hepatic sinusoids (Kupffer cells), in the lung (alveolar macrophages), brain (microglia) and skin (Langerhans cells). They are also found in the spleen, lymph nodes and bone marrow. The major roles of these cells include: phagocytosis, antigen-processing and trapping, regulation of immune responses and the secretion of several biologically important factors such as interleukin 1, lysozyme, plasminogen activator and complement components.
The macrophages of the spleen possess the ability to dispose of disintegrated erythrocytes. They do not, however, destroy hemoglobin, which is liberated in the process.
Kupffer cells, together with the cells of the general connective tissue and bone marrow, are capable of transforming into bile pigment the hemoglobin released by disintegrated erythrocytes.