Macrophages are white blood cells within tissues, produced by the division of monocytes. Human macrophages are about 21 micrometres in diameterin diameter. Monocytes and macrophages are phagocytes, acting in both non-specific defense (innate immunity) as well as to help initiate specific defense mechanisms (adaptive immunity) of vertebrate animals.
Their role is to phagocytose (engulf and then digest) cellular debris and pathogens either as stationary or as mobile cells, and to stimulate lymphocytes and other immune cells to respond to the pathogen.
They can be identified by specific expression of a number of proteins including CD14, CD11b, F4/80 (mice)/EMR1 (human), Lysozyme M, MAC-1/MAC-3 and CD68 by flow cytometry or immunohistochemical staining. They move by action of Amoeboid movement.
When a leukocyte enters damaged tissue through the endothelium of a blood vessel (a process known as the leukocyte extravasation), it undergoes a series of changes to become a macrophage. Monocytes are attracted to a damaged site by chemical substances through chemotaxis, triggered by a range of stimuli including damaged cells, pathogens and cytokines released by macrophages already at the site. At some sites such as the testis, macrophages have been shown to populate the organ through proliferation.
Unlike short-lived neutrophils, macrophages survive longer in the body up to a maximum of several months.
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