Management of Isoimmunization to Other Red Cell Antigens

MANAGEMENT OF ISOIMMUNIZATION TO OTHER RED CELL ANTIGENS

Although the routine use of Rh immune globulin has decreased isoimmunization due to the D antigen, iso-immunization due to other blood group antigens has propor-tionally increased. The frequency of these antibodies varies depending on the frequency of the antigen in the general population and in various ethnic groups. In addition, the likelihood that these antibodies will result in significant fetal hemolytic disease depends on several factors, includ-ing the size of the sensitizing antigenic stimulus, the rela-tive potency of the antigen, and the isoform (IgG or IgM) of antibody response.

Sensitization to any of these antigens can occur in any exposed women lacking the particular antigen, regardless of her ABO or Rh type. An antibody screen will detect the presence of these antibodies. The most important cause ofhemolytic disease of the fetus not associated with the D antigen is isoimmunization to the Kell antigen(see Table 19.1). Thissensitization commonly results from a prior blood transfu-sion. If a maternal antibody screen reveals the presence of an anti-Kell antibody, paternal blood typing for the Kell antigen should be performed. Because the direct phenotype of the erythrocyte for the Kell antigen and its complement—the Cellano antigen—can be performed, genotyping is not necessary. Ninety percent of individuals are Kell-negative, so if paternity is certain, no further eval-uation is required. Even among those who carry the Kell antigen, 98% are heterozygous, so consideration should be given to fetal genotype determination.

Anemia resulting from Kell isoimmunization is unique in that the predominant effect of the antibody is destruc-tion and suppression of hematopoietic precursor cells; hemolysis is only a minimal component of the fetal prob-lem. For this reason, amniotic fluid surveillance of biliru-bin may not be as useful in monitoring these pregnancies, and MCA Doppler is the preferred surveillance method. Most providers use a critical titer measurement of 1:8 to initiate further evaluation in Kell-sensitized pregnancies.

ABO hemolytic disease, due to maternal-fetal incompati-bility for the major blood group antigens, can occur. It is usuallyassociated with mild fetal and newborn hyperbilirubinemia. Typically, it is not associated with severe fetal disease, because there are fewer A and B antigenic sites on fetal red blood cells than on adult blood cells. In addition, much of the anti-A and anti-B antibody produced is of the IgM isoform that does not cross the placenta to any extent