Coloured scanning electron micrograph (SEM) of a macrophage cultured on heparin gel. 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. The picture was created as part of a cooperation with Quintessenz Publishing. Magnification: x2,000 when printed at 15cm wide.