The principle of the ABO system is that antigens – in this instance, sugars exposed on the surface of red blood cells – differ between individuals. This first discovered by Karl Landsteiner. People have immunological tolerance only for what occurs in their own bodies. As a result, humans may produce antibodies against natural components in the bodies of "other" people, but not themselves. So, people may produce antibodies against A and/or B antigens if they are not in their blood.