Macrophages (Greek: "big eaters", from makros "large" + phagein "eat") are cells of the immune system that originate from specific white blood cells called monocytes.

Monocytes and macrophages are what are known as phagocytes--immune cells that engulf and destroy pathogens, or foreign invaders. They also stimulate lymphocytes and other immune cells to respond to the pathogen.

References

"The human immune system: The lymphocyte story" (March 1988). New Scientist (1605): 1.