Monocytes are a type of white blood cell that are part of the immune system. They are produced in the bone marrow and circulate in the bloodstream. Monocytes can differentiate into macrophages, which are large phagocytic cells that engulf and digest cellular debris, foreign substances, microbes, cancer cells, and anything else that does not have the type of proteins specific to healthy body cells. Macrophages are also involved in the activation of the immune system.