Macrophage attacking bacteria. Coloured scanning electron micrograph (SEM) of a macrophage white blood cell (orange) attacking a group of Borrelia sp. bacteria (blue strings). The macrophage is engulfing the bacteria, a process known as phagocytosis. The bacteria will then be destroyed. Macrophages are white blood cells which attack foreign organisms in the body, such as bacteria. Several diseases are caused by various types of Borrelia bacteria, including Lyme disease and relapsing fever. Magnification: x4000 when printed 10cm wide.