Inhibition of anti-C1 mAb binding to fVIII by antibodies in plasmas from patients with severe hemophilia A and inhibitors. Inhibition of binding of biotinylated anti-C1 mAbs (A) 2A9, (B) F156, (C) I84, and (D) B136 to BDD fVIII in the presence of 13 inhibitor patient plasmas or fVIII-deficient plasma (severe hemophilia A noninhibitor) were determined by competitive ELISA. The corresponding normal range of control plasma ELISA titers for each mAb tested defined using EC_0.3 values within 2 SDs of the mean is represented by the gray shaded area. A test-to-control ELISA titer ratio of less than 2 SDs from the mean of the control plasma for the anti-C1 mAb tested was considered significant. Inhibitor plasmas were tested in 2 independent experiments (black dots) or 1 experiment (white dot) based on available plasma volume. Due to limited plasma volume, the ELISA titer ratio of inhibitor patient plasmas was not available for some of the anti-C1 mAbs tested as denoted by an asterisk following the inhibitor patient plasma number.