Macrophages, "big eaters", are important immune cells. They are members of the mononuclear phagocyte system. The mononuclear phagocyte system consists of blood monocytes and tissue macrophages, which are both derived from a common progenitor cell in the bone marrow. Monocytes which colonize tissue become macrophages, such as Kupffer's cells in the liver and Langerhans' cells in skin. Macrophages are the cell type most associated with chronic inflammation.