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Quantitative Precipitin Analysis of Free and Hidden RF IgM

Quantitative precipitin analysis using rabbit IgG anti‐human Fcμ was performed with 16 RF IgM, six macroglobulinaemic IgM and four normal IgM. Abnormal precipitin curves were obtained for all RF, IgM, even when the latter were not readily demonstrated with conventional serological tests for RF, and for macroglobulinaemic IgM with sedimentation rats greater than 19S. These IgM formed significantly more precipitates with IgG anti‐Fcμ in the antigen excess one than did normal IgM, but the precipitin curves for the other zones were similar for all IgM. The underlying mechanisms of some of the reactions were studied and discussed. Because the divergence in the precipitin reaction for normal IgM and RF IgM was so pronounced, a model precipitin curve was constructed. This could be used to detect RF IgM, even when not readily demonstrable with conventional serological tests for RF, by direct analysis of serum. The results obtained for RF IgM suggested that the method might be applied to RF IgG and intermediate complexes comprised of IgG. The mechanisms demonstrated here might be used to develop immunological methods for routine use.