Whenever possible, donors whoare ABO compatible with the recipient should be selected. However, sincehuman leukocyte antigen (HLA) and ABO types are unrelated, it is common tohave HLA-compatible donors who are ABO incompatible. This disparity oftenrequires special attention. Transfusion problems may occur immediately orafter a delay.

Soon after birth, antibodiesform against bacterial polysaccharides that cross-react with ABO substance.Thus, type A individuals have anti-B antibodies without ever being exposedto type B blood. Similarly, type B individuals have anti-A antibodies andtype O individuals have antibodies to both A and B, while type ABindividuals have no antibodies. Immediate transfusion risk is due to thispreformed antibody reacting with infused red blood cells (RBCs) during stemcell product infusion. Delayed reactions reflect slow turnover of plasmacells or antibody weeks or months after the transplant, resulting in delayederythroid recovery. When considering transfusion risks, both the red celltype and associated antibodies must be considered. Donor recipient pairs aredefined as follows.