The ABO blood group system is the most critical system for blood transfusions and is based on the presence or absence of specific markers, called antigens, on the surface of red blood cells (RBCs).
Antigens are substances on the red cell surface that can trigger an immune response.3 Antibodies are proteins in the plasma (the liquid part of blood) that attack foreign antigens.4
The body naturally develops antibodies against the A and B antigens that are not present on its own red blood cells.5 If an incompatible blood type is transfused, the recipient's antibodies will attack the donor's red blood cells, causing a severe, potentially fatal reaction.
Compatibility is crucial to avoid a transfusion reaction.26
Type O blood is the universal red cell donor because its red cells have no A or B antigens, so they won't be attacked by anti-A or anti-B antibodies in a recipient's plasma.27
Type AB blood is the universal red cell recipient because its plasma has no anti-A or anti-B antibodies, so it will not attack any transfused A or B antigens.28