STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
White blood cells and platelets. Scanning electron micrograph (SEM) of platelets (three smaller cells) with a variety of different white blood cells (leucocytes). Platelets, or thrombocytes, are fragments of white blood cells. When blood clots, red blood cells (erythrocytes) are trapped within a fibrin protein mesh (beige), which is formed in response to chemicals secreted by the platelets. White blood cells are involved in defending the body against both infectious disease and foreign materials. Magnification: x4500 when printed at 10 centimetres wide.
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