Activated macrophage. Coloured scanning electron micrograph (SEM) of an active macrophage white blood cell. Macrophages are cells of the body's immune system that are found in the tissues rather than in the circulating blood. They recognise foreign particles, including bacteria, pollen and dust, and phagocytose (engulf) and digest them. In the active state the cell surfaces have numerous cytoplasmic extensions (thread-like), which are used for locomotion within the tissues as the cell seeks out and engulfs the foreign particles. Magnification: x2700, when printed 10 centimetres wide.