Why is the determination of the blood types of the donor and of the recipient important in transfusions?

Red blood cells have dissimilar antigens in the outer surface of their plasma membrane; for example, the antigens A and B of the ABO system are glycoproteins of the membrane. If a donor has red blood cells with antigens not there in the red blood cells of the recipient (lacking of transfusion compatibility) the immune system of the recipient recognizes these molecules as actual antigens (i.e., foreign substances) and triggers a defense response producing exact antibodies against those antigens. The transfused red blood cells then are destroyed by these antibodies and the recipient individual might be even die.

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