Red and white blood cells, illustration. Red blood cells (erythrocytes, red) distribute oxygen to body tissues, and carry waste carbon dioxide back to the lungs. White blood cells (leucocytes, yellow-blue) are part of the immune system. They attack and destroy infectious agents (pathogens) and foreign substances, and destroy and remove cellular debris. There are different types of leucocyte, with some having a multi-lobed nucleus, and some (shown here) having a single large nucleus.