Healthy blood smear. Light micrograph of a smear of human blood consisting of a field of red blood cells (erythrocytes). Near centre is a single white blood cell (leucocyte) with a purple nucleus. Red blood cells are the most numerous type of cell in the blood. They distribute and transport oxygen and carbon dioxide around the body. Their shape is a biconcave disc, and they are flexible enough to pass through the smallest capillaries. Each red blood cell is filled with the red pigment haemoglobin which has a good oxygen carrying capacity. White blood cells serve as part of the immune system, defending the body against disease. Magnification: x630 at 35mm size.