Blood cells. Coloured scanning electron micrograph (SEM) of human red blood cells (erythrocytes, red), white blood cells (leukocytes, blue and cyan), and platelets (thrombocytes, yellow). The disc-shaped, biconcave erythrocytes transport oxygen to the body's cells and remove carbon dioxide to the lungs. Leukocytes are part of the immune system, defending the body against infection by ingesting pathogens by phagocytosis or by producing antibodies. Thrombocytes are involved with blood clotting. They gather at sites of injury and produce clotting factor. Magnification: x5000 when printed 10 centimetres high.