Molecular and immunologic characterization of the β₂GPI peptide. (A) Sequence homology between β₂GPI peptide and TLR4 (+ indicates conservative substitutions). (B) Immunoreactivity against β₂GPI peptide in patients with SLE, primary APS (PAPS), and APS secondary to SLE (SAPS) and in sex- and age-matched healthy subjects. The broken line represents the cutoff (mean + 2 SDs for healthy subjects) and the mean OD values for each group analyzed are indicated. Results are expressed as absorbance at 490 nm. (C) The reactivity of human purified anti-β₂GPI peptide and anti-BSA Abs with β₂GPI was analyzed by Western blot. Human blood–purified β₂GPI was probed with human purified anti-β₂GPI peptide (lane 1) and anti-BSA IgG (lane 2), followed by HRP-linked goat anti–human IgG Abs. 3,3′-diaminobenzidine dihydrochloride was used as a substrate.