Double precipitin arcs of IgA myeloma sera on immunoelectrophoresis. Origin and suggestion of IgA2m(2) allotype.

The serum from two patients with IgA myeloma displayed double precipitin arcs upon immunoelectrophoresis with class-specific (light chain absorbed) anti-IgA antisera obtained by immunization with monoclonal IgA1, but not IgA2 proteins. Both sera contained large amounts of monoclonal IgA2m(2) together with some polyclonal IgA1. No incomplete IgA molecules were found. The double precipitin arcs resulted from the reaction of large amounts of monoclonal IgA2m(2) with class-specific antibodies, and of small amounts of polyclonal IgA1 with subclass-specific anti-IgA1 antibodies of the same antiserum. Such double arcs, if not due to incomplete IgA molecules, could indicate the IgA2m(2) nature of the monoclonal IgA in these rare sera.