Identification of group BC/C anti-C2 antibodies in polyclonal human fVIII inhibitor plasmas. fVIII was coated on an ELISA plate and preincubated with group AB mAb I109 in both the control (severe hemophilia A noninhibitor plasma; A) and patient plasma (B) assays. Biotinylated group C Ab ESH8 was serially diluted into control plasma or inhibitor plasma. In the case of the control (C), both the group AB mAb and the biotinylated ESH8 can bind. If a saturating concentration of antibodies exists in the patient plasma that, like ESH8, bind to group BC or C epitopes, they will compete with biotinylated ESH8 for binding to fVIII (D).