Detection of β₂GPI with antibodies. (A) ELISA plates were coated with purified anti-β₂GPI IgGs isolated from APS patients. Circular or open β₂GPI was added, and binding of β₂GPI was measured with a polyclonal anti–β₂GPI antibody labeled with horseradish peroxidase. (▬) Open conformation of β₂GPI. () Circular conformation of β₂GPI. As a control for β₂GPI concentration, a mouse monoclonal anti–domain IV of β₂GPI antibody was used. Bars represent mean ± SD (n = 3). (B) ELISA plates were coated with cardiolipin, and a serial dilution (0.4-50 μg/mL) of circular (●) or open (▴) β₂GPI was added, subsequently followed by addition of purified APS patient IgG antibodies. Binding of β₂GPI was measured with a goat anti-IgG alkaline phosphatase–conjugated antibody.