Macrophage
Macrophages are part of the immune system. They are circulating scavengers. They are more highly concentrated in the blood, liver, lymph nodes and the spleen), although they are found in all tissues, especially at sites of inflamation.
While circulating through the blood stream, moving from one area of the body to another, they are called monocytes. A primary function of a macrophage is to clear foreign substances. They work by eating whatever they don't recognize as healthy tissue, including pathogens and the organism's own dead cells. In fact, the etymology of macrophage is Greek "big eater."
They present fragments of whatever they have eaten, called antigens, on their outer surface where eventually a helper T cell will notice it and release a lymphokine notification to the B cells. The B cells then create and release antibodies specific to the particular antigen, and hence to the pathogen, which will bind to the latter. Macrophages again come into play because they are especially attracted to cells with antibodies attached.
Macrophages evolve from monocytes. Monocytes evolve from the bone marrow, circulate in the blood stream for about a day and then move into tissues. In tissues they are called macrophages, etc., see below. They can survive in the tissues for several months, and in some circumstances can be triggered to divide.
Their main role is the demolition of necrotic tissue, and in the later stages of inflammation. (The early stages of inflammation are predominated by neutrophil granulocytes.)
Macrophages are sometimes known by different names depending on where they are found. This is largely for historic reasons, as originally they were not known to be the same cell. Microglia are macrophages in the brain, and Kupffer cells are macrophages in the liver.
Macrophages are the predominate cells involved in creating the progressive plaque lesions of atherosclerosis.
