Late appearance of an IgA (kappa) monoclonal protein in a patient with IgG (kappa) multiple myeloma: sharing of idiotypic specificities between the two serum proteins.

A patient with an IgG(chi) monoclonal serum protein developed in the course of the disease a second monoclonal spike of the same light chain type and of the IgA class. The latter monoclonal protein progressively increased and eventually exceeded the first IgG(chi) protein. Antigenic analysis of the two myeloma proteins demonstrated that they shared idiotypic determinants. Immunofluorescence studies, carried out with anti gamma and anti alpha reagents tagged with different fluorochromes revealed that the two isotypes were produced by different plasma cells. The data are discussed in the prospect of a possible transitional mechanism from IgG to IgA synthesis within a single B cell clone.