ABO incompatibility problems arise in two situations:
blood transfusion - the transfusion of ABO incompatible blood results in an immediate and life-threatening heamolytic transfusion reaction
in pregnancy - ABO system incompatibility may cause a mild form of haemolytic disease of the newborn. For example if the mother is blood group O and the child is blood group A, then there is a marked increase in the titre of anti-A haemolysins, but these drop back to normal levels after the pregnancy. Thus in this type of incompatibility, unlike in Rhesus disease, there is no increased risk for the next pregnancy.