Association of anti-F (ab')2 antibodies (pepsin agglutinators) with immune complexes as determined by enzyme-linked immunosorbent assays.

A solid-phase assay for the detection of anti-F(ab')2 antibodies is described. Wells of microtiter plates are coated with F(ab')2, dilutions of sera are added, and IgG bound to the solid phase is detected using peroxidase-labeled Protein A. Anti-F(ab')2 antibodies were found in 61% of sera from patients with rheumatoid arthritis, 77% with subacute bacterial endocarditis , and 34% with systemic lupus erythematosus. Simultaneous analysis of these sera for immune complexes (IC), using modified Clq and monoclonal rheumatoid factor assays, showed that a correlation existed between anti-F(ab')2 antibodies and the levels of IC. Characterization of anti-F(ab')2 antibodies by inhibition and absorption experiments and by immunological and physical means indicated that they were similar to serum proteins described in the 1960s and designated pepsin agglutinators.