The binding constants of IgM rheumatoid factors and their univalent fragments for native and aggregated human IgG;

IgM rheumatoid factors (RF) were isolated from the sera of patients with rheumatoid arthritis and a serologically active Fabmicron RF fragment prepared by papain digestion. A radioimmunoassay was developed for the determination of interaction of 19S IgM RF and Fabmicron RF with human 7S IgG, heat-aggregated IgG, rabbit 7S IgG, and human pFc'. RF isolated under neutral conditions had a very low binding constant for human 7S IgG (of the order of 10(2) to 10(3) 1 mole-1) and a considerably higher value (ca. 10(5)) for the aggregated protein and monomeric rabbit IgG. RF obtained under acid conditions which dissociate the complexes with endogenous Ig, had a higher avidity for human IgG monomer as expected and also a comparable reactivity with rabbit IgG. Monovalent Fabmicron fragments of 'acid' RF had closely similar affinities for 7S and aggregated IgG suggesting that the enhanced binding with the aggregated protein is essentially dependent on its multivalency rather than the exposure of a new determinant lacking in the native molecule.