IgM‐Rheumatoid Factors Cross‐reactive with IgG and a Cell Nuclear Antigen: Apparent ‘Masking’ in Original Serum

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Abstract
Fourteen out of fifteen IgM‐rheumatoid factor (IgM‐RF) preparations isolated from high‐titred seropositive rheumatoid sera by affinity chromatography on immobilized human IgG have been shown to cross‐react with a cell nuclear antigen, as manifest by positive immunofluorescent antinuclear antibody (ANA) staining on rat liver and kidney tissue sections. The cross‐reacting ANA activity within isolated IgM‐RF preparations was inhibitabte by both human and rabbit IgG, as well as by isolated rat liver chromatin. and could be isolated from seropositive ANA‐negative sera. It would appear that this cross‐reacting ANA activity can be masked in the original seropositive serum and may often require purification of RF from serum for its manifestation.