Characteristics of IgG antiphospholipid antibodies in patients with systemic lupus erythematosus and syphilis.

To evaluate the similarities and differences of antiphospholipid antibodies (aPL) in systemic lupus erythematosus (SLE) and syphilis, we studied 20 SLE and 16 syphilis high titer IgG aPL sera for antibody isotype, avidity, phospholipid specificity, light chain and IgG subclass. Syphilis aPL had lower avidity, with kappa light chain and IgG1 and IgG3 predominance, in contrast to higher avidity, with lambda light chain and IgG2 and IgG4 predominance in SLE aPL. Phospholipid specificities were similar. SLE and syphilis aPL both recognize phospholipid epitopes but differ in important antibody characteristics.