At some point in your life, you've probably been asked to donate blood. If your blood is type O, you may have been asked to donate even more, because your blood type is the most useful and is less common. The difference between blood types may seem small--people with blood types A and B have an extra sugar molecule bound to the surface of their red blood cells--but a transfusion of the wrong blood type can be fatal. For example, the immune system of a type O individual will launch a massive attack on the "invading cells" of a type A individual, all because it detects that sugar molecule.