A new platelet-specific alloantigen Bra. Report of 4 cases with neonatal alloimmune thrombocytopenia.

Sera obtained from 4 mothers of children with neonatal alloimmune thrombocytopenia contained a platelet-specific alloantibody, anti-Bra, which defined an antigen apparently different from all known platelet alloantigens. All 4 fathers were Bra positive, whereas all mothers were Bra negative. The minimal postnatal values of platelet counts ranged from 19 X 10(9) to 75 X 10(9)/1. Family studies showed that the Bra antigen is inherited as an autosomal, codominant trait. Its antigen frequency in the German population is 20% (21 of 105 unrelated donors were positive). The estimated gene frequency is 0.11. The antibodies were identified by a glycoprotein-specific enzyme immunoassay using monoclonal antibodies for antigen immobilization, while they could not reliably be detected by binding assays employing whole platelets (platelet immunofluorescence, indirect competitive ELISA).