Pulmonary Langerhans Cell Histiocytosis

Pulmonary Langerhans Cell Histiocytosis results from clonal proliferation of Langerhans cells (a type of dendritic cell) in the lungs, causing inflammation and progressive parenchymal destruction. Recent molecular studies have identified somatic mutations in BRAF and MAPK pathway genes in many PLCH cases, suggesting this is a clonal neoplastic process rather than purely inflammatory. The accumulation of Langerhans cells leads to nodular infiltration, ground-glass opacities, and characteristic cyst formation with upper and middle lobe predominance. The inflammatory microenvironment involves recruitment of other immune cells including eosinophils, lymphocytes, and macrophages, creating a complex pathologic milieu. Clinical presentation varies widely, from asymptomatic disease detected incidentally on imaging to severely progressive respiratory disease. Early disease may be asymptomatic or cause mild dyspnea and cough. Progressive cases show increasing respiratory compromise due to cystic lung destruction and air trapping, leading to air flow obstruction. Spontaneous pneumothorax occurs in about 15-20% of patients due to rupture of lung cysts. Some patients show spontaneous remission or slow progression over years, while others experience rapid decline requiring transplantation. High-resolution CT imaging shows characteristic findings of upper/middle lobe nodules, cysts, and ground-glass opacities. Diagnosis requires bronchoscopy with BAL showing elevated Langerhans cells or lung biopsy confirming Langerhans cell infiltration.