Monoclonal gammopathy is characterized by circulating monoclonal immunoglobulin owing to clonal proliferation of immunoglobulin-producing B lymphocytes or plasma cells. Clonal proliferation of B lymphocytes is seen in B-cell lymphoma/leukemia, and clonal plasma cell proliferation is seen in multiple myeloma and monoclonal gammopathy of undetermined significance. The monoclonal immunoglobulin in the setting of a B-cell or plasma cell disorder can cause a proliferative glomerulonephritis via 2 mechanisms: (1) glomerular deposition of the monoclonal immunoglobulin with activation of the classical pathway of complement (direct mechanism), resulting in an immunoglobulin- positive C3-positive glomerulonephritis, and (2) glomerular deposition of complement factors of the alternative and terminal pathway via inhibition of alternative pathway-regulating proteins by the monoclonal immunoglobulin (indirect mechanism), resulting in immunoglobulin-negative C3-positive glomerulonephritis (C3 glomerulopathy). Evaluation should include serum and urine electrophoresis and immunofixation as well as serum-free light-chain assay. If a monoclonal immunoglobulin is detected on these tests, bone marrow biopsy or imaging is needed to exclude more advanced plasma cell dyscrasia. Evaluation of alternative pathway of complement should be done in patients with Ig-negative C3-positive glomerulonephritis. If monoclonal gammopathy is due to an underlying malignant disease such as myeloma, lymphoma, or chronic lymphocytic leukemia, then specific treatment should be aimed at treating the malignant disease, with the goal of eradicating the clonal cells producing the immunoglobulin. In contrast, if monoclonal gammopathy is due to a monoclonal gammopathy of undetermined significance, treatment options include bortezomib, cyclophosphamide, and dexamethasone for a non- IgM monoclonal immunoglobulin and rituximab alone or in combination with cyclophosphamide and dexamethasone for an IgM monoclonal immunoglobulin.
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