macrophage A large phagocytic cell (see phagocyte) that can ingest pathogenic microorganisms (e.g. bacteria, protozoa) or cell debris and forms part of the body's immune system. Macrophages develop from precursor cells (promonocytes) in bone marrow, become wandering monocytes in the bloodstream, and then settle as mature macrophages in various tissues, including lymph nodes, connective tissues (as histiocytes), lungs (alveolar macrophages), lining of liver sinusoids (as Kupffer cells), skin (as Langerhans cells), and nervous tissues (microglia). The sinusoids of the spleen are lined with macrophages that remove worn-out red cells and platelets from the blood and destroy them. Phagocytic activity by macrophages is stimulated by macrophage-activating factor, a cytokine released by sensitized T cells. Tissue macrophages can also contribute to inflammation by secreting various cytokines. Collectively the macrophages make up the mononuclear phagocyte system.

macrophage (mak-roh-fayj) n. a large scavenger cell (see phagocyte) present in connective tissue and many major organs and tissues, including the bone marrow, spleen, lymph nodes, liver, and the central nervous system. See also histiocyte, reticuloendothelial system.

macrophage A large, phagocytic (see PHAGOCYTE; PHAGOCYTOSIS), white blood cell, which occurs in large numbers at sites of infection where it is instrumental in removing foreign cells and cell debris. Macrophages are derived from monocytes (see LEUCOCYTE) and adopt different shapes according to their location.

macrophage Large white blood cell (leucocyte) found mainly in the liver, spleen and lymph nodes. It engulfs foreign particles and microorganisms by phagocytosis, in which phagocytes ingest them. Working together with other lymphocytes, it forms part of the body's defence system.