APS sera induce C5b-9 deposition. Flow cytometry demonstrated C5b-9 (membrane attack complex) deposition on the surface of PIGA⁻ TF-1 cells in 2 representative patients: APS1 (A) and APS2 (B). Sera from both patients led to C5b-9 deposition, which was completely blocked in the presence of eculizumab (anti-C5 monoclonal antibody). Adding the factor D inhibitor ACH-4471 did not appreciably inhibit C5b-9 deposition in either patient, an observation that is also reflected in the mHam results shown in the top right of both panels. In the mHam, the dotted line at 20% nonviable cells indicates the threshold for a positive assay. (C) aHUS patient PS-18. ACH-4471 completely inhibited C5b-9 deposition induced by sera from the patient. Stx1, Shiga toxin 1 (positive control); SSC, side scatter; anti-C5 Ab, eculizumab; ACH-4471, factor D inhibitor.