Macrophages
Macrophages, seen here, are the "Big Eaters" of the immune system. They have important roles in "housekeeping" as well as starting, guiding and ending inflammatory episodes and finally also speeding up the tissue repair process by removing dead and dying tissue and immune cells. Macrophages derive from blood borne monocytes that become different types of macrophages as they migrate into different tissues where they then spend the rest of their lives. As "housekeepers" they engulf naturally dying cells to recycle their nutrients. For example, Kuppfer cells in the liver can recycle up to 3 kg of iron from red blood cells every day. Macrophages also dispose of the dying cell in a "safe" way through phagocytosis. This stops the cell content from spilling into the tissue environment where it can cause damage as well as starting an immune response unnecessarily. As tissue resident immune cells, macrophages are one of the first cells to sense and respond to invading pathogens. Substances released by pathogens attract macrophages that then phagocytose and "digest" the pathogen. Can you remember from the phagocytosis page how phagocytosis was started and two important things that then happen which alerted other immune cells and started the immune response? When macrophages recognise dying cells they do this by a protein called phosphatidyl serine which, in living cells, is found on the inside of the membrane but in dying cells is exposed on the outside. When macrophages clear naturally dying cells there is no need to alert other immune cells as there is no danger, but macrophages still secrete cytokines, but this time immune suppressive cytokines like TGF-β. This is also how macrophage resolves inflammation at the end of an immune response, by switching from producing pro-inflammatory to suppressive cytokines. Macrophages that are clearing dying cells also show digested bits of what has been taken up to T cells. Why waste energy in doing this when we don't want an immune response? Or do we, but a different kind??