ABO incompatibility

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. 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. In about a third of all ABO incompatible pregnancies maternal IgG anti-A or IgG anti-B antibodies pass through the placenta to the fetal circulation leading to a weakly positive direct Coombs test for the neonate's blood. However, ABO HDN is generally mild and short-lived and only occasionally severe because: Routine antenatal antibody screening blood tests (indirect Coombs test) do not screen for ABO HDN. If IgG anti-A or IgG anti-B antibodies are found in the pregnant woman's blood, they are not reported with the test results, because they do not correlate well with ABO HDN. Diagnosis is usually made by investigation of a newborn baby who has developed jaundice during the first week of life. Testing The antibodies in ABO HDN cause anemia due to destruction of fetal red blood cells and jaundice due to the rise in blood levels of bilirubin a by-product of hemoglobin break down. If the anemia is severe, it can be treated with a blood transfusion, however this is rarely needed. On the other hand, neonates have underdeveloped livers that are unable to process large amounts of bilirubin and a poorly developed blood-brain barrier that is unable to block bilirubin from entering the brain.This can result in kernicterus if left unchecked. If the bilirubin level is sufficiently high as to cause worry, it can be lowered via phototherapy in the first instance or an exchange transfusion if severely elevated.