Structural requirements for binding of type II anti–HPA-1a antibodies. (A) Flow cytometric analysis of the reactivity of HPA-1a–specific mAbs with human and mouse platelets. Platelets from the indicated species and having the indicated phenotypes were reacted with mAbs SZ21, 26.4, and B2G1. Note that the type II mAb 26.4 requires that muGPIIIa be humanized from Met to Gln at residue 470 of the EGF1 domain, which is spatially close to the PSI domain, as depicted in panel B. Another type II HPA-1a–specific mAb, B2G1, remains unreactive with APLDQ platelets, highlighting the complexity of binding specificities that are likely present in the polyclonal humoral response to the Leu33Pro polymorphism that controls formation of the HPA-1a epitope.