Macrophages are a type of white blood cell that begin their lives as monocytes. Monocytes are produced in the bone marrow and circulate throughout the bloodstream. When an infection or inflammation triggers a response, the monocytes can leave the blood stream and enter other tissues and organs in the body. After leaving the blood stream, monocytes develop into macrophages or dendritic cells.
Macrophages can ingest and destroy bacteria, but they can also clean up cellular debris, other harmful particles, and dead cells that contain microbes, such as bacteria or viruses. After macrophages ingest these dead cells, they will take some of the material from the microbe inside the cell and present it to other cells in the immune system. In this way, macrophages can "sound the alarm" that a foreign invader is in the body and help other immune cells recognize that invader.
Specialized, long-lived types of macrophages, called Kuppfer cells, are found in the liver. Other long-lived macrophages, called resident macrophages, can be found in the lungs, lymph nodes, spleen, certain areas of the brain, and in the kidney and bone.
Macrophages are part of the innate immune system, which means that they "non-specifically" destroy any invaders they encounter in the body, such as bacteria and parasites. Non-specifically means that macrophages do not have to recognize the invader specifically, but instead simply recognize the invader as something that should not be present and should be destroyed.