Macrophage engulfing bacteria. Coloured scanning electron micrograph (SEM) of a macrophage white blood cell (upper left) engulfing a Borrelia sp. bacterium (blue). The fine extensions of the macrophage flow round the bacteria and drag it up to the cell. This process is called phagocytosis. Macrophages are immune cells that phagocytose and destroy pathogens, dead cells and cellular debris. They are formed in the bone marrow and circulate in the blood. Magnification: x2000 when printed 10 centimetres wide.