The patient-derived anti-C1 mAb LE2E9 recognizes a group A B-cell epitope. The epitope of human mAb LE2E9 was mapped by competitive ELISA with murine anti-C1 mAbs including 4 group A mAbs (2A9, F156, I84, and M6143) and a single group B mAb (B136). For that, fVIII binding of LE2E9 used at a final concentration of 0.2 μg/mL was analyzed in the presence of increasing mAb concentrations (0.05, 0.1, 0.2, 1, 5, and 10 μg/mL). LE2E9 binding was detected using HRP-conjugated goat anti-human IgG (H+L) antibody. The experiments were repeated twice with similar results.