Russell body

**Definition:** A Russell body is a round, glassy, eosinophilic (pink) intracytoplasmic inclusion. It is found in plasma cells, and sometimes in other Ig-producing cells. A plasma cell packed with Russell bodies is called a **Mott cell** (also "morula cell" or "grape cell"). Mott cells look like a cluster of grapes.

**Pathophysiology:** - Plasma cells make large amounts of immunoglobulin on the rough ER. - If synthesis outpaces secretion, or if the Ig is misfolded or poorly secreted, it accumulates in dilated cisternae of the rough ER. - The accumulated Ig forms a dense, homogeneous globule that is PAS-positive (Ig is glycoprotein) and stains eosinophilic on H&E. - These are not true "bodies" in the sense of foreign material. They are retained, aggregated protein inside the ER.

**Key associations:** - **Multiple myeloma:** Plasma cells may contain Russell bodies (cytoplasmic), as well as Dutcher bodies (intranuclear pseudo-inclusions) and flame cells (cytoplasm with eosinophilic or red-pink staining from glycoprotein-rich Ig). - **MGUS** - **Waldenström macroglobulinemia (lymphoplasmacytic lymphoma):** IgM paraprotein. Dutcher bodies are especially characteristic. - **Chronic inflammation or infection:** Reactive plasmacytosis in chronic infections, autoimmune disease (e.g., rheumatoid arthritis), and chronic gastritis (*H. pylori*). Plasma cell–rich granulomas also show them. - **Rhinoscleroma** (*Klebsiella rhinoscleromatis*): Mikulicz cells (foamy macrophages) are the classic finding, and numerous plasma cells with Russell bodies are also seen. - **Plasma cell–rich inflammation, such as IgG4-related disease and some extranodal marginal zone lymphomas** (e.g., MALT lymphoma), can show plasmacytic differentiation with Russell bodies.

**Distinguishing from similar structures:** - **Russell body:** Cytoplasmic Ig inclusion in the dilated rough ER. Eosinophilic. - **Dutcher body:** Intranuclear pseudo-inclusion. It is actually a cytoplasmic invagination containing Ig, which appears to lie in the nucleus. It is PAS-positive. - **Mott cell:** Plasma cell with multiple Russell bodies. - **Flame cell:** Plasma cell with a flame-like, reddish cytoplasm from Ig-rich cytoplasm (classically IgA myeloma). - **Auer rods:** Needle-like azurophilic granule inclusions in myeloblasts (AML, especially APL, M3), which are myeloperoxidase-positive. They are not Ig. - **Howell-Jolly bodies:** Nuclear remnants in RBCs (asplenia).

**Clinical significance:** Russell bodies are not diagnostic of malignancy by themselves. They indicate high Ig production or impaired secretion. The diagnosis of multiple myeloma still relies on clonal plasma cells ≥10% in marrow or biopsy-proven plasmacytoma, plus an end-organ damage feature (CRAB: hypercalcemia, renal failure, anemia, bone lesions) or a biomarker of malignancy. Other findings include M spike on SPEP, rouleaux formation on smear, and Bence Jones proteins in the urine.

**High-yield pearl:** Think "Russell = Round, Red (eosinophilic), Retained Ig in the Rough ER of a plasma cell."