Macrophage attacking a foreign body. Coloured scanning electron micrograph (SEM) of a macrophage white blood cell (round, left) engulfing a Leishmania mexicana protozoan parasite (long, right). This process is called phagocytosis. Macrophages are cells of the body's immune system. They 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.