## Immunoglobulin Isotypes in Multiple Myeloma **Key Point:** IgG is the most frequent monoclonal immunoglobulin in multiple myeloma, accounting for approximately 50–60% of all cases. ### Distribution of Isotypes in MM | Isotype | Frequency | Clinical Notes | | --- | --- | --- | | IgG | 50–60% | Most common; produces hyperviscosity less frequently than IgM | | IgA | 20–25% | Second most common; higher risk of renal involvement | | IgD | 2–3% | Rare; often presents with advanced disease | | IgM | <1% | Extremely rare in MM; more typical of Waldenström macroglobulinemia | | Light chain only | 15–20% | Bence Jones proteinuria; high risk of myeloma kidney | **High-Yield:** The classic triad of MM is: (1) monoclonal protein spike on serum/urine electrophoresis, (2) bone marrow plasma cells >10%, and (3) CRAB criteria (Calcium elevation, Renal insufficiency, Anemia, Bone lesions). IgG is the most frequent culprit in this triad. **Clinical Pearl:** Although IgG is most common overall, IgA myeloma has a higher propensity for renal damage due to its larger molecular size and propensity to form polymers, leading to cast nephropathy (myeloma kidney). **Mnemonic:** **GOAD** — IgG is the most common (remember "G" for "Gold standard" of MM), followed by IgA, then Others (IgD, IgM), then light chain-only (D for "Dimerized" or "Dimer-prone").
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