Binding of type A IgG antibodies to β₂-GPI in solution. Hydrophilic ELISA plates were coated with 10 μg/mL plasma-purified β₂-GPI, and patient type A IgG antibodies were incubated to the plate in the presence of various concentrations of plasma-purified β₂-GPI and recombinant β₂-GPI. The patient type A IgG antibodies bound to the plate were detected by using an alkaline-phosphatase-labeled goat anti-human IgG antibody. PnPP was used as coloring substance. The reaction was stopped by the addition of 2.4 M NaOH, and absorbance was measured at 405 nm. The obtained optical density was corrected for the optical density obtained with total IgG isolated from pooled plasma from 40 healthy volunteers (OD, 0.168).