Langerhans cell histiocytosis (LCH) is characterized by clonal proliferation of Langerhans cells bearing the BRAF V600E mutation. Which feature on electron microscopy is pathognomonic for Langerhans cell identification?

• A Birbeck granules (tennis racket-shaped intracytoplasmic inclusions) ✓
• B Weibel-Palade bodies
• C Dense core granules (neurosecretory granules)
• D Lamellar bodies (surfactant bodies)

Explanation

Birbeck granules are unique rod-shaped or tennis-racket-shaped membrane-bound structures seen on EM of Langerhans cells; they represent internalized Langerin (CD207) receptor complexes involved in antigen capture. Immunohistochemically, LCH cells are CD1a+, CD207 (Langerin)+, and S100+. Weibel-Palade bodies are seen in endothelial cells (contain vWF and P-selectin). Dense core granules are seen in neuroendocrine cells. Lamellar bodies are seen in type II pneumocytes (surfactant secretion).

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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