Detection of IgM autoantibodies in eluates from red blood cells.

Forty-two patients with autoimmune haemolysis, whose red cells were sensitized with IgM, were studied in order to determine suitable techniques for investigating the form of erythrocyte-bound IgM, particular emphasis being placed on identifying forms of low molecular weight. Four elution techniques were assessed for their efficiency in removing cell-bound IgM; these were the heat elution, acid stromal, organic solvent and freeze-thaw methods. Eluted IgM was detected by polyacrylamide gel electrophoresis followed by immunoblotting; the heat elution method proved to be the best. Although oligomeric forms of IgM were found in 64% of patients' sera tested concurrently, only pentameric IgM was eluted; from this it was concluded that low molecular weight IgM was not causing haemolysis in any of the cases studied but that the methods would readily detect its presence.