Macrophage attacking a foreign body. Coloured scanning electron micrograph (SEM) of a macrophage white blood cell (round, centre) with two Leishmania mexicana protozoan parasites (blue). Macrophages are cells of the body's immune system. They engulf (phagocytose) and destroy pathogens, dead cells and cellular debris. Leishmania mexicana is a single-celled organism that causes a form of the disease known as leishmeniasis.