Complement activation by MBB2. Human β2GPI (10 μg/mL) immobilized on γ-irradiated polystyrene plates was first incubated with either MBB2 (1 µg/mL) or control MB (1 µg/mL) or IgG aPL⁺ isolated from a patient with APS for 90 minutes at room temperature and then with 1:100 fresh human serum for 30 minutes at 37°C. The bound C1q (A), C4 (B), and C9 (C) were identified by ELISA using goat antibodies to C1q and C4 and the monoclonal antibody aE11 to C9 neoantigen. The data were analyzed with the Mann-Whitney test. The experiments were performed in triplicate, and the data are expressed as mean ± SD, *P < .05 vs control MB.