A new assay for IgG rheumatoid factor activity and its use to analyse rheumatoid factor reactivity with human IgG isotypes.

A new assay for IgGRF activity is described which employs human IgGFc as the antigen and a radiolabelled monoclonal antibody directed against human IgG (CHI domain) as the developing antibody. Using this assay IgGRF activity against human IgG isotypes was measured and most sera from RA patients were shown to react predominantly with IgG1 and IgG2 but few reacted with IgG3 and IgG4. The same sera were tested for IgMRF to the IgG isotypes. IgG2 was the best antigen, IgG1 and IgG4 were less so and reactivity with IgG3 was the lowest. IgGRF without associated IgMRF was obtained, its specificity compared to that of IgMRF, and found to be broadly similar. With the new assay high levels of serum IgGRF were found in those RA patients with extraarticular disease but not in RA patients with synovitis alone.