A review of histiocytosis X with particular reference to eosinophilic granuloma of the lung.

Electron microscopy confirms the unity of Histiocytosis X (HX) by identifying a common marker organelle in all three forms of the disease (acute disseminated HX or Letterer-Siwe disease; chronic disseminated HX or Hand-Schuller-Christian disease; localized HX or eosinophilic granuloma). The marker organelle of HX is also found in normal Langerhans' cells: these mononuclear cells are distinct from macrophages but are similarly concerned in antigen transport. HX may therefore be considered to represent a pathological proliferation of Langerhans' cells. The natural history and the clinical, radiological and pathological features of primary pulmonary HX (eosinophilic granuloma of the lungs) are described, together with the differential diagnoses which must be considered by the histopathologist.