Macrophages are a type of immune cell that, along with mast cells, provides the front-line cellular defense of the immune system. Whereas mast cells are generally located at the barriers that surround tissues, macrophages are embedded within body tissues.
For historical reasons, macrophages have different names based on the part of the body that they normally live in. Macrophages that normally live in the blood are called monocytes. Macrophages that normally live in the skin are called Langerhans cells. Macrophages that normally live in the liver are called Kupffer cells. And macrophages that normally live in the central nervous system are called microglia. When scientists refer simply to "macrophages", they generally refer to blood-borne monocytes that have left the blood and entered a tissue.