In ABO hemolytic disease of the newborn (also known as ABO HDN) maternal IgG antibodies with specificity for the ABO blood group system pass through the placenta to the fetal circulation where they can cause hemolysis of fetal red blood cells which can lead to fetal anemia and HDN. In contrast to Rh disease, about half of the cases of ABO HDN occur in a firstborn baby and ABO HDN does not become more severe after further pregnancies. The ABO blood group system is the best known surface antigen system, expressed on a wide variety of human cells. For Caucasian populations about one fifth of all pregnancies have ABO incompatibility between the fetus and the mother, but only a very small minority develop symptomatic ABO HDN. The latter typically only occurs in mothers of blood group O, because they can produce enough IgG antibodies to cause hemolysis. Although very uncommon, cases of ABO HDN have been reported in infants born to mothers with blood groups A and B. Presentation Complications High at birth or rapidly rising bilirubin Prolonged hyperbilirubinemia Bilirubin Induced Neurological Dysfunction Cerebral Palsy Kernicterus Neutropenia Thrombocytopenia Hemolytic Anemia - MUST NOT be treated with iron Late onset anemia - Must NOT be treated with iron. Can persist up to 12 weeks after birth. Causes Environmental exposure Anti-A and anti-B antibodies are usually IgM and do not pass through the placenta, but some mothers "naturally" have IgG anti-A or IgG anti-B antibodies, which can pass through the placenta. Exposure to A-antigens and B-antigens, which are both widespread in nature, usually leads to the production of IgM anti-A and IgM anti-B antibodies but occasionally IgG antibodies are produced. Fetal-maternal transfusion Some mothers may be sensitized by fetal-maternal transfusion of ABO incompatible red blood and produce immune IgG antibodies against the antigen they do not have and their baby does. For example, when a mother of genotype OO (blood group O) carries a fetus of genotype AO (blood group A) she may produce IgG anti-A antibodies. The father will either have blood group A, with genotype AA or AO or, more rarely, have blood group AB, with genotype AB. Blood transfusion It would be very rare for ABO sensitization to be due to therapeutic blood transfusion as a great deal of effort and checking is done to ensure that blood is ABO compatible between the recipient and the donor. Risk factors