The prevalence of HPA-1a alloimmunization and the potential risk of FNAIT depends on both the DRB3*01:01 allele and associated DR-DQ haplotypes.

Alloimmunization against human platelet antigen (HPA)-1a during pregnancy can cause foetal/neonatal alloimmune thrombocytopenia (FNAIT) and severe bleeding in the foetus or newborn and likely depends on several factors. HPA-1a alloimmunization is associated with DRB3*01:01, which is associated with several DR-DQ haplotypes. However, it is not known to what extent these haplotypes contribute to the prevalence of HPA-1a alloimmunization. HPA-1a-alloimmunized women, identified in a prospective study, and random donors were typed for selected DRB3, DRB4, DRB1, DQA1 and DQB1 alleles to determine allele and DR-DQ haplotype frequencies. DRB3*01:01 was carried by 94% HPA-1a-immunized women compared to 27% in the general population. In the first population, the DR3-DQ2 haplotype was overrepresented (P < .003). The prevalence of HPA-1a alloimmunization was estimated to be about twice as frequent with DR3-DQ2 compared to DR13-DQ6, together accounting for about 90% of DRB3*01:01-positive individuals. Further, we examined DQB1*02 and DRB4*01:01 alleles for their reported association with HPA-1a alloimmunization, in the context of DR-DQ haplotypes. Since similar to 80% of DQB1*02 alleles are linked to the DR3-DQ2 haplotype, the association might be coincidental. However, the DQB1*02:02-associated DR7-DQ2 haplotype was also overrepresented in alloimmunized women, suggesting a role for this allele or haplotype in HPA-1a alloimmunization. As DRB4*01:01 is predominantly associated with the DR7-DQ2 haplotype in HPA-1a-alloimmunized individuals, the reported association with FNAIT may be coincidental. Typing for DR-DQ haplotypes revealed important genetic associations with HPA-1a alloimmunization not evident from typing individual alleles, and the presence of different DRB3-associated DR-DQ haplotypes showed different prevalence of HPA-1a alloimmunization.