92 shalid, you fail to properly answer the question. You see, red blood cells have tracer markers on them, The A,B,0 relates to one of them. A and B refer to two different tracers. You can have one, both, or neither, this is determined by genetics. If you have A, you have one kind. If you have B, the other. AB is both tracers while O is negative. Then their is the Rhesus factor, named so because it was first discovered in Rhesus monkeys. This is the positive or negative. If positive, you have the rhesus tracer, if negative you dont. Then their is in application of blood donors. The immune system attacks foreign bodies by recognition of these tracers, which are present on most living cells, both human and otherwise. Human red blood cells, lacking in a nucleous and other major organelles have different characteristices. If your blood is O-, than your body does not recognize any such tracers as natural. So if blood with a tracer on the cells is in you, a serious autoimmune reaction occurs, your blood clumps in you and you die. If your blood is AB+, your body recognizes all the tracers and thus any blood can go in you, for the immune system does not recognize the absence of a tracer, only the absence.