Classic diseases revisited Langerhans cell histiocytosis (histiocytosis X)

Accepted 29 March 1996 In 1953, Lichtenstein introduced the unifying term 'histiocytosis X' to embrace several previous eponyms, including eosinophilic granuloma, Letterer-Siwe disease and Hand-Schuller-Christian disease.' In 1987, a more prominent designation, Langerhans cell histiocytosis (LCH), was recommended to replace histiocytosis X.' The term is more appropriate as it reflects the fact that the Langerhans cell is the main cell involved in these several diseases. The Langerhans cell is a subtype of histiocyte first described by Dr Paul Langerhans in 1868.3 LCH is a rare disease arising from pathological proliferation of Langerhans histiocytes. Histiocytes are formed in the bone marrow from a pluripotent self-renewing haemopoietic stem cell.4'5 They consist of two groups, namely mononuclear phagocytes (ordinary histiocytes) and antigen-presenting histiocytes. The former group includes the migratory blood monocyte and the fixed tissue macrophage (such as the hepatic Kuffer cell, pulmonary alveolar macrophage, brain microglia and splenic sinusoidal histiocyte). The latter group has a dendritic morphology which comprises the lymph nodal interdigitating reticulum cell and dendritic reticulum cell, the lymphoid dendritic cells of the blood and the Langerhans cell. The Langerhans cell is normally found in the skin. It can also occasionally be identified in many tissues such as in the bronchi, oral mucosa, oesophagus, thymus, etc. They are the most potent antigen-presenting cells in the body. In 1987, the Histiocyte Society proposed three main classes as the working classification for histiocytoses,6 which gained widespread acceptance until 1994, when Pritchard and Broadbent put forward a modified version of the classification, dividing histiocytoses into four main classes (box 1).5