Red and white blood cells. Coloured scanning electron micrograph (SEM) showing two types of cells found in human blood. In the background is a red blood cell (erythrocyte) and in the foreground is a white blood cell (leucocyte). Red blood cells are biconcave disc- shaped cells which transport oxygen around the body. Their red colour comes from the protein haemoglobin. Each cubic milli- metre of blood contains around 5 million of these most numerous blood cells. The white blood cell is roughly spherical with microvilli projecting from its surface. Different types of white cells are active in the body's immune response to infection. Magnification: x6,000 at 6x7cm size.