Platelet anti-Bakb antibody associated with neonatal alloimmune thrombocytopenia

A platelet alloantibody of IgG class but unknown specificity was detected in the serum from the mother of an infant with neonatal alloimmune thrombocytopenia. The antibody reacted by indirect immunofluorescence with 33/65 random donors, 10/11 Baka-negative donors and 23/54 Baka-positive donors. The mother's platelet phenotype was PLA1, Koa, Kob, Baka and the father's was PLA1, Koa, Kob, Baka. Immunoblotting and immunoprecipitation of maternal serum with paternal platelets produced a band at molecular weight 140 kilodaltons identical to the band obtained with Baka antiserum. Family studies confirmed the allelic distribution of Baka and the unknown antigen. The platelet-specific antibody in this patient meets the criteria for an antibody to the new platelet antigen, Bakb.