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Immunofluorescent evidence for cellular control of synthesis of variable regions of light and heavy chains of immunoglobulins G and M by the same gene.

Two distinct paraproteins (IgG(2)-K and IgM-K) from one patient shared identical light chains and significant portions of the variable regions of the heavy chains. Idiotypic determinants on the IgG and IgM molecules were shared. Earlier studies, using class-specific antisera, showed that these paraproteins were produced by two different populations of cells. The present study, using rhodamine and fluorescein conjugates of the anti-idiotype antisera, demonstrates that all plasma cells that contain immunoglobulin, whether IgG or IgM, stained with anti-idiotype antisera; this occurred irrespective of whether the antisera were made originally against the patient's IgG or IgM. This, plus previous data, indicates that both populations of cells share genetic information for the constant and variable regions of light chains and significant portions of the heavy-chain genes. These, and other cited data, strongly suggest the occurrence of a switch-over from IgM to IgG synthesis in the same cell during the course of the normal immune response.