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Immunohistochemical and Ultrastructural Examination of Histiocytosis X Cells in Pulmonary Eosinophilic Granuloma

We describe histological, immunohistochemical, and ultra‐structural findings in pulmonary eosinophilic granuloma (PEG) from three patients. The specimens were taken by open‐lung biopsy. The lesions of the lung were composed of histiocytic cells, macrophages and eosinophils. The histiocytic cells reacted positively with anti‐S‐100 protein antibody. The histiocytic cells had various types of Birbeck granules in the cytoplasm. The histiocytic cells were histiocytosis X (HX) cells considered to be derived from Langerhans cells. Sporadic mitosis of HX cells was observed. Some HX cells had migrated from the lesions into the alveolar spaces through the alveolar cell layers. In the lesions of PEG, HX cells have self‐reproduction and migration capability. Acta Pathol Jpn 42: 719–726, 1992.