Which immunoglobulin class is most commonly produced as the monoclonal protein in the most common type of multiple myeloma?

• A. IgG
• B. IgA
• C. IgM
• D. IgD

Answer: (A)

In multiple myeloma, a single clone of malignant plasma cells produces a homogeneous immunoglobulin, called the M-protein. The most common heavy-chain class seen in this monoclonal protein is IgG, which is why IgG monoclonal gammopathy accounts for the largest share of myeloma cases. This reflects that IgG is the most prevalent immunoglobulin produced by plasma cells in normal conditions, so when a malignant clone expands, its favored product is IgG. The IgG M-protein is typically detected as a sharp spike on serum protein electrophoresis and then confirmed by immunofixation showing an IgG heavy chain with a specific light chain (kappa or lambda). Other classes exist but are less common in myeloma: IgA is the next most frequent, while IgM is more characteristic of other gammopathies like Waldenström macroglobulinemia, and IgD myeloma is quite rare.