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  1. Definition. Isoimmune hemolytic anemia may result when ABO incompatibility occurs between the mother and the newborn infant. This disorder is most common with blood type A or B infants born to type O mothers. The hemolytic process begins in utero and is the result of active placental transport of maternal isoantibody. In type O mothers, isoantibody is predominantly 7S-IgG (immunoglobulin G) and is capable of crossing the placental membranes. Because of its larger size, the mostly 19S-IgM (immunoglobulin M) isoantibody found in type A or type B mothers cannot cross. Symptomatic clinical disease, which usually does not present until after birth, is a compensated mild hemolytic anemia with reticulocytosis, microspherocytosis, and early-onset unconjugated hyperbilirubinemia.

  2. Incidence. Risk factors for ABO incompatibility are present in 12–15% of pregnancies, but evidence of fetal sensitization (positive direct Coombs test) occurs in only 3–4%. Symptomatic ABO hemolytic disease occurs in <1% of all newborn infants but accounts for approximately two thirds of observed cases of hemolytic disease in the newborn.

  3. Pathophysiology. Transplacental transport of maternal isoantibody results in an immune reaction with the A or B antigen on fetal erythrocytes, which produces characteristic microspherocytes. This process eventually results in complete extravascular hemolysis of the end-stage spherocyte. The ongoing hemolysis is balanced by compensatory reticulocytosis and shortening of the cell cycle time, so that there is overall maintenance of the erythrocyte indices within physiologic limits. A paucity of A or B antigenic sites on the fetal (in contrast to the adult) erythrocytes and competitive binding of isoantibody to myriad other antigenic sites in other tissues may explain the often mild hemolytic process that occurs and the usual absence of progressive disease with subsequent pregnancies.

  4. Risk factors

      1. A1 antigen in the infant. Of the major blood group antigens, the A1 antigen has the greatest antigenicity and is associated with a greater risk of symptomatic disease. However the hemolytic activity of anti-B antibodies is higher than those of anti-A and may produce a more severe disease in particular among infants of African American descent.

      1. Elevated isohemagglutinins. Antepartum intestinal parasitism or third-trimester immunization with tetanus toxoid or pneumococcal vaccine may stimulate isoantibody titer to A or B antigens.

      1. Birth order. Birth order is not considered a risk factor. Maternal isoantibody exists naturally and is independent of prior exposure to incompatible fetal blood group antigens. First-born infants have a 40–50% risk for symptomatic disease. Progressive severity of the hemolytic process in succeeding pregnancies is a rare phenomenon.

  5. Clinical presentation

      1. Jaundice. Icterus is often the sole physical manifestation of ABO incompatibility with a clinically significant level of hemolysis. The onset is usually within the first 24 h of life. The jaundice evolves at a faster rate over the early neonatal period than nonhemolytic physiologic pattern jaundice.

      1. Anemia. Because of the effectiveness of compensation by reticulocytosis in response to the ongoing mild hemolytic process, erythrocyte indices are maintained within a physiologic range that is normal for asymptomatic infants of the same gestational age. Additional signs of clinical disease (eg, hepatosplenomegaly or hydrops fetalis) are extremely unusual but may be seen with a more progressive hemolytic process ...

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