Pulmonary langerhans cell histiocytosis: Historical context and current concepts

Pulmonary Langerhans cell histiocytosis (PLCH) is a rare interstitial lung disease seen as part of multisystem Langerhans cell histiocytosis, or as an isolated form of the disease. The majority of cases of the latter are seen in adults and associated with cigarette smoking, which probably plays a central etiologic role. Although the mechanisms leading to the development of isolated PLCH are unclear, it is thought to be a reactive process in contrast to other forms of Langerhans cell histiocytosis, which have been shown to represent clonal, neoplastic proliferations of Langerhans cells. Isolated PLCH and the other forms may be altogether separate diseases, but they have in common great variability in severity and, likewise, unpredictable clinical courses. In this article, we provide an historical context for these elusive diseases, as well as summarize the most recent literature on PLCH.