SPECIFICITY OF RHEUMATOID FACTORS CROSS‐REACTING WITH HUMAN AND RABBIT IgG

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Abstract
The reaction of rheumatoid factors with rabbit IgG sensitized erythrocytes was inhibited by Fc fragments and by a mixture of Facb and pFc' fragments of rabbit IgG, but not by either of the latter two fragments alone. The reaction was also inhibited by heat aggregated human IgG1 and IgG2 myeloma proteins, depending on the amount of aggregates present, but irrespective of their Gm types. No inhibition was seen with IgG3 or IgG4 proteins. Immunosorbent studies showed that the antigen of human IgG involved in these reactions is probably closely related to the γ1–2‐4 antigen, which is a major human IgG antigen interacting with rheumatoid factors. Quantitative haemagglutination inhibition studies in an Auto‐Analyzer showed that native human IgG very weakly inhibited the reaction of rheumatoid sera with rabbit IgG. Isolated aggregated human IgG inhibited far better, and gave an inhibition curve indicating the formation in aggregated human IgG of new antigens very similar to antigens of native rabbit IgG. Thus, in rheumatoid arthritis there may be a humoral immune response also against antigens present in aggregated but not in native human IgG, giving the cross‐reaction with native rabbit IgG.