Radioimmunoassays for anti-F(ab')z antibodies, which feature the use of goat anti-human Fc antibody for correcting potentiation of IgM anti-F(ab')z antibody titers by endogenous IgM anti-Fc antibodies (rheumatoid factors), are described. Individuals with classic rheumatoid arthritis had significantly more IgM anti-F(ab')2 antibody (P < 0.001) and IgG anti-F(ab')2 antibody (P = 0.05) than did individuals with systemic lupus erythematosus or normal volunteers. There is some similarity in patterns of isotype distribution of anti-F(ab')z antibodies and rheumatoid factors.

The specificity of IgG anti‐F(ab′)2 antibodies was examined in unfractionated sera of patients with rheumatoid arthritis and also with affinity‐purified antibody preparations. Examination of the sera by an enzyme‐linked immunosorbent assay, using pooled human F(ab′)2 fragments absorbed to microtiter plates, revealed that IgG anti‐F(ab′)2 antibodies cross‐react with human and rabbit IgG and rabbit F(ab′)2. IgG anti‐F(ab′)2 antibodies were purified by affinity chromatography and, when tested by a fluid‐phase inhibition enzyme‐linked immunosorbent assay, were found to be of 2 types. One fraction, similar to pepsin agglutinator, reacted with human F(ab′)2 fragment alone. The other fraction was cross‐reactive with human IgG and yet failed to react with idiotopes on Fab or epitopes on Fc fragments. The IgG anti‐F(ab′)2 antibodies we purified had no reactivity toward a human immune complex prepared from tetanus toxoid and antitoxoid.

An isotope specific immunoassay which minimizes interference by endogenous rheumatoid factors was used to determine the specificity of IgM anti-F(ab')2 antibodies in human serum. We underscore the heterogeneity of these antibodies. While one subset of IgM anti-F(ab')2 antibodies reacts only with intact F(ab')2, another recognizes determinants present following reduction and alkylation of F(ab')2 and separation of Fd' fragments from light chains. IgM anti-F(ab')2 antibodies in sera from rheumatoid arthritis patients do not react significantly with intact pooled IgG and, therefore, probably are not anti-idiotypic antibodies. Some sera, but not all, contain elevated levels of antibodies that are crossreactive with rabbit F(ab')2. Such crossreactive antibodies may interfere with assays which utilize F(ab')2 fragments of rabbit antibodies specific for antigens of clinical relevance.