Microglia are macrophages derived from bone marrow. They migrate to the central nervous system during development where they become trapped after the blood brain barrier forms. Microglia are normally inactive but become activated in response to antigenic stimulation. Once activated, they proliferate and migrate to the site of injury where they bind to antigens by using the molecules on their surface. They produce the toxic cytokins IL-1 and TNFalphan when activated. They are major phagocytes, but are located in the central nervous system.