Preferential rheumatoid factor reactivity with disulphide bond altered IgG in radioimmunoassay.

Rheumatoid factor (RF) antibody populations were purified by immunoabsorption from rheumatoid arthritis patients' sera and from rabbit hyperimmune anti-streptococcal sera. On the basis of the particular affinity matrix from which the RF were eluted, the antibody populations were classified as being preferentially reactive with either mildly reduced and alkylated (MRA) or native, intact, homologous (with regard to species) IgG. Immune complexes formed between these RF preparations and IgG were characterized by their susceptibility to polyethylene glycol (PEG) precipitation. The two RF specificity populations were incubated in the presence of 125I-labelled MRA and intact homologous IgG preparations. The resultant complexes were subsequently precipitated in the presence of 0-20% (w/v) PEG. From the data generated by incubation of a constant amount of RF in the presence of either intact or MRA IgG, the amount of complex precipitated over the range of PEG concentrations examined was greatest when the form of IgG used for immunoadsorption was also used as the radiolabelled antigen. When a constant concentration of PEG was used and the concentration of the IgG antigen and RF were separately varied, precipitable complexes were formed at lower concentrations of each variable when radiolabelled antigen homology, with respect to the affinity matrix, was maintained. Therefore, although cross-reactive, the two RF specificities were clearly selective with regard to their affinity for a given form of IgG.