HLA-DRB1 and thrombotic thrombocytopenic purpura: Whereas there are accumulating data about the role of the DRB1*11 risk allele in the pathogenesis of TTP through efficient presentation of certain ADAMTS13-derived peptides by the DR11 molecule (22, 23, 42, 43), the mechanism by which the protective haplotypes lower the risk of developing TTP is to date obscure.