The Bra/Brb alloantigen system on human platelets.

Anti-Bra was first identified in four cases of neonatal alloimmune thrombocytopenia (NAIT). The antigen Bra is localized on the glycoprotein Ia/IIa complex of platelets. Anti-Bra can best be detected by a glycoprotein-specific immunoassay using monoclonal antibodies for antigen immobilization (MAIPA assay) and radioimmunoprecipitation (RIP). Recently, we have identified sera from two polytransfused patients that contain an antibody that recognizes Brb, the allele of Bra. Family studies show that both antigens are inherited as autosomal codominant characters. The gene frequency of the new allele Brb is 0.888. Approximately 2,000 anti-Bra binding sites are present on homozygous platelets and 1,000 on heterozygous platelets. Our findings provide evidence for the first polymorphism observed on the glycoprotein Ia/IIa complex. Immunization against these alloantigens is implicated in NAIT and poly-transfused patients.