Macrophage. Coloured scanning electron micrograph (SEM) of a macrophage (brown) engulfing Staphylococcus sp. bacteria (orange). Macrophages are scavenging white blood cells (leucocytes) present in many body tissues. As part of the body's immune response, they accumulate at sites of infection and engulf foreign invaders by a process known as phagocytosis. They also play an important role in stimulating other cells of the immune system to respond to foreign agents. Staphylococcus sp. are significant human pathogens that cause skin infections and food poisoning. Magnification: x1400 at 6x7cm size.