Autoantibodies bind to the C-terminus of FH. (A) Reactivity of autoantibodies of 5 aHUS patients was tested with recombinant FH fragments, covering all domains of FH. BSA was used as negative control and FH as positive control. Normal human serum was included as additional control. Data are representative of 3 experiments with similar results. (B) Autoantibody binding to FH was blocked with mAbs (25 μg/mL) specific to the C-terminal domains SCR19-20 of FH (ie, C02, C14, and C18), whereas mAbs N11, M12, M13, and M15, which bind in the N-terminal and middle regions of FH, did not inhibit autoantibody binding. Representative data of 3 experiments are shown. (C) Binding domains of the identified FH autoantibodies and the mAbs used in this study. Important functional domains of FH are highlighted.