Pathogenic anti-β₂GPI IgG induces C5b-9 deposition. (A) Patient APS21, anti-β2GPI IgG. Flow cytometry demonstrated that IgG anti-β2GPI antibody from patient APS21 with thrombotic APS induced C5b-9 (membrane attack complex) deposition on the surface of PIGA⁻ cells. C5b-9 deposition was blocked completely with eculizumab and partially by the factor D inhibitor ACH-4471. Patient APS21 was a 30-year-old woman with venous thrombosis and recurrent pulmonary hemorrhage. aPL profile: LA⁺, aCL IgG⁺, aCL IgM⁻, anti-β2GPI IgG⁺, and anti-β2GPI IgM⁻. (B) Patient APS3, anti-β2 GP1 IgM. The IgM led to a very small increase in C5b-9 deposition. APS3 was a 51-year-old woman with persistently positive aPLs, but no thrombosis. aPL profile: LA⁺, aCL IgG⁻, aCL IgM⁺, anti-β2GPI IgG⁻, and anti-β2GPI IgM⁺. NHS(H), heat-inactivated NHS; Stx1, Shiga toxin 1 (positive control); SSC, side scatter; anti-C5 Ab, eculizumab; ACH-4471, factor D inhibitor.