Two M-components in a single cell lineage in a patient with a dual isotype secretory B-cell tumour.

We describe a case of Waldenström's macroglobulinaemia with two M-components (IgM and IgG) with the same lambda light chain. Southern blot analysis of bone marrow cells showed rearrangements of immunoglobulin heavy and lambda light chain genes. Sequencing of the complementarity determining region 3 of the two lambda and mu transcripts showed 100% homology. Immunofluorescence study showed that most cells stained for both IgG and IgM. These findings indicated that a single population of cells was expressing two isotypic variants of IgG and IgM, as the genes responsible for production of both components had the same origin.