Inhibition of intrinsic Xase activity by anti-C2 MAbs. FVIII (1 nM) was incubated for 2 hours at 37°C in the absence (closed symbols) or presence (open symbols) of 10 μg/mL VWF and in the absence (○) or presence of 20 nM anti-C2 MAbs: 3E6 (group A) (▿), 1B5 (group B) (◇), 2-77 (group BC) (▵), or 2-117 (group C) (□). Then fVIII was activated rapidly by thrombin (100 nM) for 30 seconds, followed by addition of desulfatohirudin (150 nM) to inhibit thrombin. At this concentration of thrombin, fVIII is activated completely within 10 seconds.²⁷  The fVIIIa sample was added to 2 nM factor IXa/20 μM PCPS, followed immediately by addition of 300 nM factor X and measurement of the initial velocity of factor X activation as described in “Intrinsic fXase assays for measurement of fVIIIa activity.”