Hemolytic Transfusion Reaction Due to Anti-A₁ Antibody During Pregnancy: Case Report

The ABO blood group is the most clinically relevant system in transfusion medicine. Approximately 20% of individuals with blood group A of European descent belong to a weak A subgroup, most commonly A₂, which may produce anti-A₁ antibodies. These antibodies are usually cold-reactive IgM and rarely cause hemolysis, but can occasionally be clinically significant when reactive at 37°C. We describe a pregnant woman with sickle cell disease (HbS/β⁰ thalassemia) and prior hyperhemolysis syndrome who developed a severe delayed hemolytic transfusion reaction (DHTR) after transfusion of A₁ red blood cells (RBCs). Anti-A₁ was identified posttransfusion, confirming her as a non-A₁ subtype. Notably, she also experienced hemolysis following group O red cell transfusion, consistent with hyperhemolysis. This case highlights the rare but serious potential of anti-A₁ to cause DHTR, particularly in high-risk populations, and underscores the importance of increased vigilance when managing transfusion in sickle cell disease.