Background:　Blood　coagulation　factor　VIIa　(FVIIa)　plays　its　critical　physiological　role　in　the　initiation　of　hemostasis.　Even　so,　recombinant　FVIIa　is　successfully　used　as　a　bypassing　agent　for　factor　VIII　or　IX　in　the　treatment　of　bleeds　in　patients　with　severe　hemophilia　with　inhibitors.　To　investigate　the　utility　of　more　potent　FVIIa　variants　with　enhanced　intrinsic　activity,　molecules　such　as　V21D/E154V/M156Q-FVIIa　(FVIIa(DVQ))　were　designed.　Methods:　Surface　plasmon　resonance　was　used　to　characterize　the　binding　of　mAb4F5　to　FVIIa(DVQ)　and　related　variants.　X-ray　crystallography　was　used　to　determine　the　structure　of　the　Fab　fragment　of　mAb4F5　(Fab4F5).　Molecular　docking　and　small-angle　X-ray　scattering　led　to　a　model　of　FVIIa(DVQ):Fab4F5　complex.　Results:　The　binding　experiments,　functional　effects　on　FVIIa(DVQ)　and　structure　of　mAb4F5　(originally　intended　for　quantification　of　FVIIa(DVQ)　in　samples　containing　FVII(a))　pinpointed　the　epitope　(crucial　role　for　residue　Asp21)　and　shed　light　on　the　role　of　the　N-terminus　of　the　protease　domain　in　FVIIa　allostery.　The　potential　antigen-combining　sites　are　composed　of　1　hydrophobic　and　1　negatively　charged　pocket　formed　by　6　complementarity-determining　region　(CDR)　loops.　Structural　analysis　of　Fab4F5　shows　that　the　epitope　interacts　with　the　periphery　of　the　hydrophobic　pocket　and　provides　insights　into　the　molecular　basis　of　mAb4F5　recognition　and　tight　binding　of　FVIIa(DVQ).　Conclusion:　The　binary　complex　explains　and　supports　the　selectivity　and　functional　consequences　of　Fab4F5　association　with　FVIIa(DVQ)　and　illustrates　the　potentially　unique　antigenicity　of　this　FVIIa　variant.　This　will　be　useful　in　the　design　of　less　immunogenic　variants.