Monoclonal gammopathies are B cell neoplasms that are characterized by the presence of monoclonal immunoglobulins (M-proteins) in the serum. By an unknown mechanism, the normal polyclonal immunoglobulin levels are frequently reduced in sera of these patients. To assess the role of M-protein isotype in this effect, we used serum protein electrophoresis to quantitate monoclonal and polyclonal immunoglobulins in patients and we used serum immunofixation electrophoresis to determine their M-protein

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... isotype. When divided into populations of 30 patients with IgG M-proteins (mean 2.5 g/dl) and 19 patients with IgM or IgA M-proteins (mean 2.6 g/dl), the mean polyclonal immunoglobulin level was significantly lower in the IgG M-protein population (0.4 g/dl) than the IgM/IgA population (0.8 g/dl). Patients with IgG M-proteins also had significantly lower polyclonal immunoglobulin levels when compared separately with the patients with either IgA or IgM paraproteins. Since the polyclonal immunoglobulin fraction is comprised mostly of IgG, these results give the first direct indication that IgG M-proteins have a greater suppressive effect on polyclonal IgG levels than do M-proteins of other isotypes. These findings suggest that an isotype-specific feedback mechanism could be involved in the normal regulation of serum IgG levels.