Receptors for IgM on human lymphocytes. III. Specificity of receptors.

Receptors for the Fc portion of IgM (RFcmu) can be detected on a large percentage of human peripheral lymphocytes. To approach an understanding of the function of these receptors we have examined the relative efficiencies of different preparations of human IgM and their F(c)5 mu fragments in binding to RFcmu. This was measured by blocking rosette formation between lymphocytes and IgM-sensitized ox erythrocytes (EAmu). IgM from pooled normal human serum completed blocked formation of IgM rosettes (EAmu-RFC) at much lower concentrations than IgM preparations from individual Waldenström's macroglobulinaemic patients (WM). The differences in effectiveness of these IgM preparations in inhibiting EAmu-RFC suggested the existence of receptors for subclasses of IgM on different subpopulations of human lymphocytes. F(c)5 mu fragments of IgM were more effective in blocking EAmu-RFC than the parent molecule, which was in turn more efficient than the 8S subunit. These results indicate that F(c)5 mu may have more Fc regions exposed per molecule for binding to the matrix of RFcmu than pentameric IgM and that Fc multivalency may be important for the stable binding of IgM to RFcmu.