Depending on the antigen on the red blood cells the blood type is classified by two elements: The ABO grouping system and the Rhesus (Rh) factor which is either present (Rh positive) or not present (Rh negative). For example, if you have A antigens on your red blood cells, your blood type is A. If someone has both A and B antigens, they are type AB. If you do not have any antigens on the surface of red blood cells, your blood type is O. For each antigen on the blood cell, the opposite antibody is produced in the plasma. For example, type B blood has anti-type A antibodies. Receiving blood that’s incompatible with one’s own blood type may trigger a serious immune response that can be fatal. While there are more than 20 blood-type systems, ABO and Rh are by far the most important.