Posttransfusion purpura associated with an anti-Bak.

We describe a 46-year-old white woman with typical clinical features of posttransfusion purpura (PTP) whose serum held a platelet-specific alloantibody reactive with an antigen antithetical to Baka, i.e. anti-Bakb. The specificity of the antibody was confirmed by family analysis, a population study (expected versus observed gene frequency: 0.3651 versus 0.3984; n = 105) and localization of the antigen on glycoprotein IIb in radioimmunoprecipitation. Typing of family members and blood donors for platelet antigens disclosed that the patient had been preimmunized by two blood transfusions in 1981, while fetomaternal incompatibility for Bakb was ruled out (her three children and their father were Bakb negative). Treatment of PTP with corticosteroids and platelet transfusions was ineffective, but infusion of high-dose intravenous IgG resulted in a rapid increase in the platelet count.