IgG titer, subclass, and light‐chain phenotype of pregnancy‐induced HPA‐5b antibodies that cause or do not cause neonatal alloimmune thrombocytopenia

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Abstract
BACKGROUND: The most common pregnancy‐induced platelet‐specific antibody is HPA‐5b. Neonatal alloimmune thrombocytopenia that results from anti‐HPA‐5b may cause severe hemorrhage in only a few infants, but the sequelae for the affected children can be severe. It is therefore essential that infants at risk for neonatal alloimmune thrombocytopenia are identified. STUDY DESIGN AND METHODS: The IgG titer, subclass, and light‐chain composition of pregnancy‐induced anti‐HPA‐5b were determined by the monoclonal antibody‐specific immobilization of platelet antigens assay. Sera were from 12 mothers, who among them had 16 pregnancies that resulted in an HPA‐5‐mismatched fetus (positive for HPA‐5b). Eight mothers gave birth to an infant with a normal platelet count. Three maternal sera were obtained after delivery of a severely thrombocytopenic infant. Three alloimmunized women were followed repeatedly during the course of a subsequent pregnancy, again with an HPA‐5‐mismatched infant. RESULTS: There was no difference in the antibody titer or its subclass in mothers who had a thrombocytopenic child and the titer or subclass in mothers compared with those who had a child with a normal platelet count. All pregnancy‐induced HPA‐5b antibodies were of a predominant, lambda, light‐chain type. The IgG subclass did not change during pregnancy. CONCLUSION: Neither the antibody titer nor the subclass composition predict the occurrence of thrombocytopenia in a newborn whose mother is alloimmunized against HPA‐5b.