Neutralization of the inhibitory anti–factor V antibodies from patient RB by recombinant human factor V C2 domain. Purified IgG from patient RB (≈5 μg/mL) was incubated with purified recombinant factor V C2 domain at the concentrations shown for 15 minutes at 37°C. Conditioned media containing rHFV des B (≈0.5 nmol/L) was then incubated with either the inhibitor IgG/C2 domain mixture (•) or the same concentration of the C2 domain alone (○) for 15 minutes at 37°C. Residual activity was then determined by the chromogenic prothrombinase assay, as described in Materials and Methods.