Electrophoretic and immunologic differences between monoclonal IgM molecules and their subunits in native plasma

Naturally occurring IgM subunits had a higher negative charge than those obtained after mercaptoethanol reduction of 19 S IgM and higher polymers. Both types of subunits regularly had a stronger negative charge than the pentamers. Extension of the immunoelectrophoretic IgM precipitation line on the anodal side of the 19 S M-component component indicated occurrence of subunits if trailing could be excluded. The precipitation patterns were easier to interpret when electroendoosmotic inert support media were used for electrophoresis. In most cases antigen-antibody crossed electrophoresis allowed semiquantitative estimation of proportions between mono- and pentamers. Of 32 sera with increased monoclonal IgM, 25 were found to contain measurable amounts of 7 S IgM. Quantitative precipitin and gel diffusion studies showed that maximal amounts of precipitates with anti-IgM were obtained in antigen excess for 19 S IgM, but an equivalence for 7 S IgM.