Blood cells. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes, red) and a white blood cell (leucocyte, grey). The disc- shaped erythrocytes transport oxygen to the body cells and carry waste carbon dioxide to the lungs. The leucocyte is part of the immune system, which defends the body against infection by digesting pathogens or secreting antibodies. Magnification: x3400 at 6x7cm size.