Erythroblastosis fetalis and hemolytic disease of the newborn (HDN) are caused by maternal anti-fetal erythrocyte antibodies, which cause a type II hypersensitivity response leading to erythrocyte destruction.
The implicated maternal antibodies are of the IgG subtype, as these are the only class of antibody that is readily able to cross the placenta.
With maternal blood types A and B, isoimmunization does NOT occur as the naturally occurring antibodies (anti-A and -B) are of the IgM type, which cannot cross the placenta.
In contrast, type O mothers who have antibodies with IgG subtype that can cross the placenta trigger hemolysis in the fetus. That is why HDN is mainly seen in a group O mother who has a group A or B baby.
Unlike Rh alloimmunization reactions, ABO incompatibility reactions can occur in the 1st pregnancy because both A and B antigens are found in food and bacteria in the environment.
Transfusion remains a key treatment for individuals with haemoglobinopathies, particularly in sickle cell disease, and this remains complicated by the significant risk of red cell alloimmunisation
Allergic reactions occur most commonly with plasma rich components. E.g. platelets, FFP and cryoprecipitate.