Recognition of human IgM subgroups by quantitative measurement.

By means of the single radial immunodiffusion technique it is possible to detect at least two subgroups of human IgM paraproteins, if the square ring diameter is plotted against absolute concentration. These groups give different calibration lines with the same antiserum, even when no qualitative differences can be detected by the double diffusion technique. The possibility of distinguishing these groups is not limited to the use of a single antiserum. The differences between the groups are abolished by reduction of the IgM paraprotein to 7S subunits and therefore seem to be of conformational nature. No identity with other known classifications has yet been found. In one out of two cases investigated differences in quantitative reactivity were found between 19S and >19S components in the same serum. The results suggest that quantitative methods may be used to detect differences in immunoglobulin structure, when qualitative methods fail. They show that different human IgM paraproteins cannot be directly compared quantitatively by the radial immunodiffusion method.