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Macrophages are important cells of the immune system that are formed in response to an infection or accumulating damaged or dead cells. A macrophage has the ability to locate and `eat` particles such as bacteria, viruses, fungi, and parasites. Macrophages are born from white blood cells called monocytes, which are produced by stem cells in our bone marrow. In biochemistry and pharmacology, a receptor is a protein molecule that receives chemical signals from outside a cell. When such chemical signals bind to a receptor, they cause some form of cellular/tissue response, e.g. a change in the electrical activity of a cell.