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How can incompatible blood transfusions cause hemolytic anemia?

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blood disease Peripheral blood smear revealing histopathologic features indicative of a blast crisis in a case of chronic myelogenous leukemia (CML). Blast crisis occurs when one-fifth to one-third of the cells in the blood or bone marrow are immature blood cells, or blast cells. This phase of CML is characterized by fever, weakness, and an enlarged spleen.

Incompatible blood transfusions cause hemolytic anemia through an immunologic reaction where the recipient's antibodies recognize and attack the donor's red blood cells as foreign substances.12

When donor blood contains antigens that are incompatible with the recipient's blood type—such as in the ABO blood group system—the recipient's immune system triggers a rapid response. Specifically, if a recipient has pre-existing antibodies against the donor's antigens (for example, a person with type B blood receiving type A blood), these antibodies bind to the donor's red cells. This binding leads to lysis, or the destruction of the donor's red blood cells.1342

This process results in an intravascular hemolytic reaction, where the rapid destruction of red cells releases free hemoglobin into the circulation. If the rate of red cell destruction exceeds the body's ability to produce new cells, the patient develops hemolytic anemia. Beyond the ABO system, similar reactions can occur with other blood groups, such as the Rh blood group system or the Kell group, particularly if a patient has been sensitized through previous transfusions.124