These cells are professional phagocytes. The migrate to sites of inflammation or infection to eat, digest and destroy invading microorganisms. During infection, the body makes huge numbers of these cells, accounting for the elevated WBC counts commonly seen during serious infections. These cells contain lots of histamine and other mediators of inflammation. Mast cells in tissues are very similar to basophils in blood.
Monocytes are white blood cells that become macrophages when they migrate into the tissues.
Found fixed in tissues and migrating through them, macrophages are sentry cells of the immune system. They detect microorganisms and signal other leukocytes to begin making an immune response. They are phagocytic cells. Tissue injury or infection sets off inflammation. The process of inflammation works to protect against infection.