Maternal anti-HPA-1a levels (IU/mL) in relation to dose of the HLA-DRB3*01:01 allele. Antibody levels from HLA-DRB3*01:01–negative women, who carried the HLA-DRB3*02:02 allele, are marked with filled circles. Women who were heterozygous or hemizygous (1 dose) had higher antibody levels than HPA-1a–immunized HLA-DRB3*01:01–negative women (0 dose), but they had lower antibody levels than women who were HLA-DRB3*01:01 homozygous (2 doses). The horizontal bars denote the median values in each group of women. Kruskal-Wallis test was used to calculate P_global. The other P values were calculated by using the Mann-Whitney U test.