## Immunoglobulin Isotypes in Multiple Myeloma **Key Point:** IgG myeloma accounts for approximately 50–60% of all multiple myeloma cases, making it the most common isotype. ### Distribution of Myeloma by Isotype | Isotype | Frequency | Clinical Notes | |---------|-----------|----------------| | IgG | 50–60% | Most common; associated with lytic bone lesions | | IgA | 20–25% | Second most common; higher incidence of renal involvement | | IgD | 2–3% | Rare; often presents with advanced disease and extramedullary involvement | | IgM | <1% | Extremely rare in myeloma; more common in Waldenström macroglobulinemia | | Light chain only | 15–20% | Bence Jones proteinuria; higher risk of renal failure | **High-Yield:** The CRAB criteria (Calcium elevation, Renal insufficiency, Anemia, Bone lesions) are independent of isotype, but IgG myelomas classically present with lytic bone disease due to the large size and osteoclast-activating properties of IgG immune complexes. **Clinical Pearl:** IgD myelomas, though rare, are associated with a worse prognosis and more aggressive clinical course compared to IgG and IgA myelomas. They frequently present with extramedullary disease and higher serum calcium levels. **Mnemonic:** **GIG** — **G**amma (IgG) is the **G**reatest in myeloma; **I**mmune (IgA) is **I**ncreased second; **D**ifficult (IgD) is **D**isease when present.
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