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In the early 20th century constellations of symptoms now attributed to Langerhans cell histiocytosis (LCH) were first recognized as Hand-Schuller-Christian disease, Letterer-Siwe disease, and eosinophilic granuloma. Lichtenstein then proposed the unifying nosology "Histiocytosis X," named after the unknown origin of the proliferating cell. The disease was finally renamed LCH after Nezelof proposed the "X" may in fact be the Langerhans cell (LC), which he associated with the Birbeck granule. 1 The conditions defined by LCH lie on a clinical spectrum. The term histiocytosis represents a range of histiocytopathies derived from the mononuclear phagocyte system, namely monocytes, tissue-resident macrophages, and dendritic cells. The dendritic cell (DC), an antigen-presenting cell, can be further categorized as LC, myeloid DC, plasmacytoid DC, resident perivascular factor XIIIA DC, and inflammatory DC. 2 The neoplastic proliferation of LCs manifests as LCH. LCH is recognized phenotypically by a CD4+, S100+, CD1a+, and langerin (CD207+) profile in dendritic cells amidst an inflammatory background. 1 LCs can also be recognized on electron microscopy by the presence of Birbeck granules, within which the langerin protein is localized. Phenotypic analyses reveal both immature and mature (indeterminate) LC proliferations, with maturation defined by the loss of Birbeck granules and langerin as the cell migrates from skin to lymph node.

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