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How can incompatible ABO blood cause a hemolytic transfusion reaction?

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Blood transfusion — blood transfusion Depiction of a direct blood transfusion performed in 1882.

A hemolytic transfusion reaction occurs when there is an incompatibility in the ABO blood group system, leading to a rapid immune response within the recipient's blood vessels.1

This reaction is triggered when a patient receives blood containing antigens that do not match their own. For example, if a person with type A blood—who naturally possesses antibodies against type B red cells—is transfused with type B blood, their immune system identifies the donor's red blood cells as foreign. The recipient's antibodies bind to these donor cells, causing lysis (destruction) of the red blood cells.23

This process, known as an intravascular hemolytic reaction, can be life-threatening and is often the result of errors such as patient misidentification, mislabeled blood components, or mistakes in blood sample processing. Because the destruction of red cells occurs rapidly within the circulatory system, it can lead to severe clinical complications and, in some cases, death.13