The compositional difference between blood types lies in the specific kinds of antigens found on the surface of the red blood cells. Antigens are protein molecules that provide the biological mark of an individual's blood and tissue type. When different types of blood are mixed within the body, the reaction can be fatal as red blood cells burst and the kidneys fail. Today, there are 27 known human blood groups. Understanding blood compatibility issues has had a revolutionary effect on the field of medicine.