Blood stream. Animation of cells in the lumen of a blood vessel. Seen here are red and white blood cells and antibodies (y-shaped). Red blood cells are biconcave, giving them a large surface area for gas exchange. Their main function is to distribute oxygen to body tissues and to carry waste carbon dioxide back to the lungs. White blood cells are part of the body's immune system. They perform functions ranging from cleaning up cellular debris, to defending the body against bacteria and other pathogens. Antibodies bind to specific antigens, for instance viral proteins. In doing this they mark the antigen for destruction by phagocytes, white blood cells that ingest and destroy foreign bodies. Antibodies can also kill some pathogens directly and can neutralise toxins.