Rheumatoid factor and serum IgG, IgM and IgA levels in rheumatoid arthritis with vasculitis.

The authors report a significant increase in the rheumatoid factor titre in rheumatoid arthritis patients with vasculitis. A significant rise in IgG and IgA levels was found in uncomplicated RA, when compared with a normal population. The IgM levels were not found to be elevated in this group of RA patients. In the RA patients with vasculitis, on the contrary, the three Ig classes are increased when compared with the normal population, and the IgM level is increased when compared with the uncomplicated RA group. Significant relation was found between log IgM and log R.F. titre in the RA group with vasculitis. It is shown that the mild reduction of serum (by dithiothreitol 0.004 M) causes a complete negativity of the R. F. in all cases. It is concluded that the haemagglutination and the latex precipitation in vitro are induced by IgM-R. F. and not by IgG and IgA-R. F. molecules. Only free sites of IgM-R.F. play a functional role in the determination of the R. F. -titre. The authors postulated that increase in IgM in RA patients with vasculitis is partially due to the presence of free IgM-R. F. molecules in serum. Finally it is concluded that hidden IgM-R. F. molecules in patients with lower R. F. titre are not quantified by immunodiffusion methods.