Blood cells in a capillary, artwork. Red blood cells (erythrocytes, red) distribute oxygen to body tissues, and carry waste carbon dioxide back to the lungs. White blood cells (leucocytes, white) are part of the immune system. They attack and destroy infectious agents (pathogens) and foreign substances, and destroy and remove cellular debris. Platelets (thrombocytes, yellow) are active in blood coagulation (clotting).