The H3 anti‐phospholipid idiotype is found in patients with systemic lupus erythematosus (SLE) but not in patients with syphilis

SUMMARY The human H3 idiotype. defined by a mouse monoclonal antibody S2.9, is commonly found in patients with SLE where it is correlated with the amount of anti‐cardiolipin antibodies. No correlation between the amount of anti‐cardiolipin antibody and the H3 idiotype is found in patients with syphilis. Using the S2.9 antibody, serum from each of 10 patients with SLE and eight patients with syphilis was separated into H3‐bearing and H3‐negative fractions. Comparison of the partition of anti‐cardiolipin antibody in these two groups of patients revealed that much of the anti‐cardiolipin antibody (44–91%) was found in the H3 + fraction in patients with SLE; in patients with syphilis. virtually none of the anti‐cardiolipin antibody was H3 + . In patients with SLE. the H3 + fraction contained both IgG and IgM and antibodies of both kappa and lambda light chains. The H3 + fraction was polyspecific and frequently reacted with dsDNA.