Histiocytosis X refers to a group of diseases of abnormal proliferation of histiocytes not caused by infection or abnormal lipid metabolism, and is denoted by X because the cause is unknown. Some scholars believe that the disease is caused by granulomas formed by Langerhans cells transformed from dendritic cells that invade the lungs and bones, including three diseases that are pathologically similar and have different clinical manifestations, age of onset, course and prognosis: Le-Sue disease, Han-Sue-Ko disease and pulmonary eosinophilic granuloma (now called Langerhans granuloma). Their common feature is the abnormal proliferation of Langerhans cells. The first two diseases are mostly seen in children under 5 years of age, with multi-organ damage, high mortality rate and poor prognosis; Langerhans granulomatosis occurs in adults, preferably between 20 and 40 years of age, with more males than females, and mainly affects lung and bone. Clinical and pathological features: abnormal proliferation of histiocytes in the lung is called pulmonary histiocytosis X, also known as pulmonary eosinophilic granuloma, a rare disease of unknown cause characterized by pulmonary histiocyte infiltration. Patients mostly have a history of smoking, and common symptoms are chronic cough and mild dyspnea, which may be accompanied by chest pain and spontaneous pneumothorax when the pulmonary vesicles rupture. Some patients may have mild to moderate fever, hemoptysis, night sweats, loss of appetite, and weight loss. Patients may also have no respiratory symptoms or mild symptoms that cannot be taken seriously, and no positive pulmonary signs are found only during chest X-ray examination. Lung function is dominated by restrictive ventilatory dysfunction and decreased diffusion function. Confirmation of the diagnosis requires a lung tissue biopsy. Pathology shows: granulomatous inflammation with a high number of Langerhans histiocytes within the granuloma and also a significant eosinophil infiltration; immunohistochemical staining shows: Langerhans cells S-100 positive, consistent with Langerhans histiocytosis (eosinophilic granulomatosis). Treatment and prognosis: There is no specific treatment for PHX. PHX is usually chronic, most patients are asymptomatic for a long time, and the prognosis is good; some patients can recover spontaneously, but those with multiple organ involvement have a poor prognosis. Imaging features: diffuse but predominantly small nodules (mostly less than 5 mm), patchy, and cystic shadows in both lungs, with irregular or round-like cavities and thick cystic walls; small nodules predominate in the early stage and cystic cavities predominate in the later stage (cystic walls develop from thick to thin) – cellular lung. Lung volume remains normal; mediastinal and hilar lymph node enlargement is rare. 25% of patients may have pneumothorax as a complication. Differential diagnosis: 1. Pulmonary lymphangioleiomyosarcoma (PLAM): it occurs in women of childbearing age, with clinical manifestations: progressive worsening of cough and dyspnea, repeatedly complicated by pneumothorax. Imaging manifestations: uniformly distributed thin-walled cysts of varying sizes throughout the lung, no obvious nodal shadow, and no narrowing of the lung field. The mortality rate is high and the prognosis is poor. In contrast, PHX occurs in males aged 20-40 years, with mild clinical symptoms and frequent pneumothorax, and significant dyspnea when developing to fibrosis stage and cellular lung. Some patients may resolve on their own, and the prognosis is good. 2, pulmonary nodular disease: both respiratory symptoms and systemic symptoms are very mild or asymptomatic, early both have the possibility of self-remission or healing, both are diffuse shadows, but the lung volume is not reduced. CT manifestations of nodular disease: the shadow distribution is relatively uniform, with the upper and middle lung lesions being obvious, and most of them are accompanied by symmetrical hilar lymph node enlargement on both sides, and other organs are often involved at the same time. If there is skin and superficial lymph node involvement, biopsy can be diagnosed. PHX lesions are confined to the lungs, and there are no positive experimental findings, so lung biopsy must be relied on to confirm the diagnosis. 3, idiopathic interstitial lung fibrosis (IPF): Although both are confined to the lungs, the clinical symptoms and prognosis are very different, both have diffuse shadows, but PHX early for small dots, lamellar shadows mixed, more uniform distribution, less fibrosis, no significant reduction in lung volume. In contrast, IPF shadows first appear in the middle and lower lung field zone, and the lesions are concentrated in the middle and lower lungs, causing the lower lungs to shrink, the hilum to drop and approach the mediastinum, and the lesions continue to worsen, forming a honeycomb lung in the late stage, with significant reduction in lung volume and elevation of the diaphragm. The former has mild symptoms and a tendency to heal spontaneously, while the latter continues to deteriorate, with progressive dyspnea from early onset, pestle-like fingers, and velcro sounds often heard in the back of both lungs. Both diagnoses rely on lung biopsy. 4.Alveolar cell carcinoma: early clinical symptoms are mild, with the development of the disease, coughing, coughing a lot of white foamy sputum, dyspnea, shadows can occur in one lung in the early stage, and then gradually develop to the opposite side. In contrast, PHX starts with symmetrical shadows, and although there are many shadows, the clinical symptoms are mild. Cancer cells can be found in the sputum of alveolar cell carcinoma. Both of them can be diagnosed by lung biopsy.