New insights into the pathogenesis of Langerhans cell histiocytosis.

Langerhans cell histiocytosis (LCH) is an enigmatic disease, usually occurring in the young. Despite the fact that clinical entities of the disease have been known for some time, the pathogenesis and etiology remain obscure. A major stride toward understanding LCH was taken when ultrastructural and immunohistochemical studies identified the "histiocytic" cells in LCH lesions to be members of the Langerhans (dendritic) cell system. Another finding of importance was the discovery that LCH cells within the lesions are clonal. Clonality alone, however, is not enough to define LCH as a neoplasm. In this article, we review the recent developments in the pathogenesis and etiology of LCH and discuss the implications of these findings.