Clinical meaning
Pulmonary Langerhans cell histiocytosis (PLCH) is characterized by clonal proliferation of Langerhans cells (specialized dendritic cells bearing CD1a and CD207/langerin markers) that infiltrate the small airways and interstitium. Cigarette smoke antigens are believed to trigger this abnormal immune response. The Langerhans cells form peribronchiolar granulomas that destroy airway walls and adjacent alveoli. Early lesions are cellular and inflammatory; mature lesions become fibrotic and form stellate scars. End-stage disease features cystic destruction of lung parenchyma and can lead to pulmonary hypertension.
Exam relevance
Risk factors: - Cigarette smoking (> 90% of cases) - Age 20-40 years - White ethnicity - BRAF V600E mutation in Langerhans cells - History of other smoking-related lung diseases
Diagnostics: - High-resolution CT showing upper/middle lobe cysts and nodules (pathognomonic pattern) - BAL showing > 5% CD1a-positive cells (suggestive) - Surgical lung biopsy showing Langerhans cell granulomas with stellate scarring - Pulmonary function tests showing mixed obstructive-restrictive pattern - DLCO typically reduced - Bone scan or PET scan to rule out multisystem involvement