Structure of factor VIII and reported epitopes of factor VIII inhibitors. Epitope regions are indicated by arrows. In this case, the dominant IgG subclass is IgG4, and the binding region is the 72-kDa A3/C1/C2 domain of the light chain. FIXa and FXa indicate activated factor IX and X, respectively. Abbreviations: VWF, von Willebrand factor; APC, activated protein C.