Why is the determination of the blood types of the donor and of the recipient important in transfusions?
The Red blood cells have different antigens in the outer surface of their plasma membrane; for instance, the antigens A and B of the ABO system are glycoproteins of the membrane. If a donor has red blood cells with the antigens not present in the red blood cells of the recipient (lacking of transfusion compatibility) the immune system of the recipient recognizes these molecules as actual antigens (that is foreign substances) and triggers a defense response producing specific antibodies against those antigens and the transfused red blood cells then are destroyed by these antibodies and the recipient individual may even die.