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Why must donor and recipient blood types be matched before a transfusion?

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Blood plasma — blood Blood sediment in test tubes, showing plasma (clear fluid), red blood cells (red), and the release of hemoglobin into the surrounding plasma (pink).

Matching donor and recipient blood types is essential to prevent a potentially fatal immune response known as a hemolytic transfusion reaction.12

This process is critical because individuals possess specific antigens on their red blood cells and corresponding antibodies in their serum. If incompatible blood is transfused, the recipient’s antibodies will identify the donor's red cells as foreign and attack them, causing the cells to clump together (agglutination) and undergo lysis (destruction).1345

The most significant systems for this matching are the ABO blood group system and the Rh system. For example, a person with type A blood has anti-B antibodies; if they receive type B blood, their immune system will destroy the donor cells. Similarly, in the Rh system, transfusing Rh-positive blood into an Rh-negative individual can trigger the production of anti-D antibodies, which may cause severe complications in future transfusions or during pregnancy.145

To ensure safety, medical professionals perform compatibility tests, such as the direct match test, to confirm that the recipient’s serum does not contain antibodies that will react against the donor’s cells.15