Macrophages are cells produced by the differentiation of monocytes in tissues. Macrophages were discovered by Ilya Mechnikov, a Russian bacteriologist, in 1884. Human macrophages are about 21 micrometres (0. 00083 in) in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense (innate immunity) as well as help initiate specific defense mechanisms (adaptive immunity) of vertebrate animals. Their role is to phagocytose, or engulf and then digest, cellular debris and pathogens, either as stationary or as mobile cells. They also stimulate lymphocytes and other immune cells to respond to pathogens. They are specialized phagocytic cells that attack foreign substances, infectious microbes and cancer cells through destruction and ingestion. Macrophages can be identified by specific expression of a number of proteins including CD14, CD40, 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.