[The Br(a)/Br(b) alloantigen system in thrombocytes].

Platelet specific alloantibodies cause neonatal alloimmune thrombocytopenia (NAIT), posttransfusion purpura (PTP) and may be found in patients who are refractory to HLA-matched platelet transfusion. Most platelet alloantigen systems comprise two alleles: Zw(a)/Zw(b), Ko(a)/Ko(b), Bak(a)/Bak(b), Yuk(a)/Yuk/b). The corresponding antibodies are detected with the platelet immunofluorescence test, radioimmunoprecipitation, immunoblot, and with a glycoprotein-specific immunoassay. Epitopes of the Zw-, Bak-, and Yuk-alloantigen systems are localized on the GPIIb/IIIa complex. Recently, we characterized antibodies against two alleles of a new alloantigen system (Bra/Brb). Anti-Bra is the second most important antibody to cause NAIT, following Anti-Zwa. Anti-Brb was found in three polytransfused patients. The Br-antibodies may be detected using a glycoprotein-specific enzyme immunoassay (MAIPA assay) and radioimmunoprecipitation. The frequency of the Br-phenotypes in our population is Br(a+b-) 1%, Br(a+b+) 20%, Br(a-b+) 79%. The number of binding sites for Anti-Bra is 2000/platelet on homozygous and 1000/platelet on heterozygous platelets. The Br-alloantigens are localized on GPIa, which is identical with the alpha chain of VLA-2 on activated T-lymphocytes.