Severe hemolytic transfusion reaction due to anti-A1 following allogeneic stem cell transplantation with minor ABO incompatibility.

Blood components should be compatible both with the recipient and the donor in the ABO incompatible allogeneic stem cell transplantation setting. A patient with blood type A2 received peripheral blood stem cells from a blood type O donor. The patient was in critical condition due to treatment-related toxicity. He had acquired anti-A1 that was unfortunately overlooked. Following transfusion of A1 red blood cells in error, he developed a severe hemolytic transfusion reaction. Anti-A1 is rarely clinically significant. We discuss the role of passenger lymphocytes in development of the anti-A1, and stress the importance of investigating unusual/atypical reactions in blood typing.