Molecular forms of IgA rheumatoid factor in serum and synovial fluid of patients with rheumatoid arthritis

The distribution of molecular forms of serum IgA rheumatoid factor (IgA-RF) in 42 patients with rheumatoid arthritis was examined by solid-phase radioimmunoassay following fractionation by gel chromatography or ultracentrifugation in acidic buffer. Analysis of the fractions using phosphate buffered saline indicated that the IgA-RF in each serum was mainly polymeric. However, monomeric IgA-RF was detected in sera from approximately two-thirds of the patients, after dilution of chromatographic or ultracentrifugal fractions in diluent containing mouse monoclonal anti-human alpha chain antibody. The levels of monomeric IgA-RF (mean +/- SD 38 +/- 86 micrograms/ml) and the ratios of monomeric to polymeric IgA-RF (mean +/- SD 0.29 +/- 0.41) varied over a wide range. Paired synovial fluids from 9 of the patients were also examined. Monomeric IgA-RF was detected in each, although 2 samples demonstrated only minimal quantities. Neither form of IgA-RF was detected in serum from healthy adults when analyzed under the same conditions. Thus, both monomeric and polymeric IgA-RF can occur in serum and synovial fluid from patients with rheumatoid arthritis, and their proportions vary widely among patients.