Langerhans cell histiocytosis
Formerly known as Histiocytosis X. It refers to a spectrum of abnormal clonal proliferations of dendritic cells and now includes the diseases previously known as eosinophilic granuloma, Hand-Schüller-Christian disease and Letterer-Siwe disease.
The histiocytes originate in the bone marrow, but due to the nature of the cells are able to infiltrate various organs. The clinical behaviour can be variable, varying from aggressive and ultimately fatal disease to more indolent, self-resolving lesions.
There is some debate whether it represents a neoplastic lesion, a reactive process or is due to immune system dysregulation.
Usually found in a mixed, eosinophil-rich inflammatory cell background. The histiocytes are mononuclear and have a reniform nuclei. Immunohistochemical staining is typically positive for CD1a, S100, CD4, CD45 and langerin (CD207), but not CD68.
Electron microscopy reveals Birbeck granules.