The macrophage sometimes originates in the bone marrow. Known as monocytes in their immature form, these cells leave the marrow, travel through the bloodstream for a few days, and then migrate into the tissues.
There they mature into macrophages, or “big eaters”, professional scavengers with a nearly insatiable appetite for undesirable cells. Many macrophages settle at the common site of entry for undesired microorganisms, including the tissues of the lungs, digestive system and circulatory system. When a macrophage receives a warning signal from infected cells or tissues, the innate intelligence of the body commands that macrophages develop still further, acquiring even more sophisticated cellular machinery.
Amazing isn’t it?