Plasma-derived (pd) or recombinant (r) therapeutic FVIII proteins (FVIIIs) are infused to arrest/prevent bleeding in patients with hemophilia A (PWHA). However, FVIIIs are neutralized if anti-FVIII-antibodies ("inhibitors") develop. Accumulating evidence suggests that pdFVIIIs with von Willebrand factor (VWF) are less immunogenic than rFVIIIs and that distinct rFVIIIs are differentially immunogenic. Since inhibitor development is T-helper-cell-dependent, HLA-class-II (HLAcII) molecules comprise an important early determinant.Use dendritic cell (DC)-protein processing/presentation assays with mass-spectrometric and peptide-proteomic analyses to quantify the DP-, DQ-, and DR-bound/FVIII-derived-peptides in individual HLAcII repertoires and compare the immunogenic potential of six distinct FVIIIs based on their measured peptide counts.Monocyte-derived-DCs from normal donors and/or PWHA were cultured with either: "Mix-rFVIII", a VWF-free equimolar mixture of a full-length (FL)-rFVIII (Advate®) and four distinct B-domain-deleted (BDD)-rFVIIIs (Xyntha®, Novoeight®, Nuwiq®, and Afstyla®); a pdFVIII + pdVWF (Beriate®); Advate ± pdVWF; Afstyla ± pdVWF; and Xyntha + pdVWF.We showed that: (i) Beriate had a significantly lower immunogenic potential than Advate ± pdVWF, Afstyla - pdVWF, and Mix-rFVIII; (ii) distinct FVIIIs differed significantly in their immunogenic potential in that, in addition to "(i)", Afstyla + pdVWF had a significantly lower immunogenic potential than Beriate, while the immunogenic potential of Beriate was not significantly different from Xyntha + pdVWF; and (iii) rFVIIIs with pdVWF had a significantly lower immunogenic potential than the same rFVIIIs without pdVWF.Our results provide HLAcII peptidomic level explanations for several important clinical observations/issues including the differential immunogenicity of distinct FVIIIs and the role of HLAcII genetics in inhibitor development.