Case Report: IgG4-related lung disease.

Immunoglobulin G4-related disease (IgG4-RD) is a systemic immune-mediated fibroinflammatory condition, distinct from classical autoimmune disorders, that can affect multiple organs. Pulmonary involvement often presents with non-specific clinical and radiographic features, contributing to high rates of misdiagnosis and underdiagnosis. Here, we report a rare case of IgG4-related lung disease (IgG4-RLD) to enhance clinical awareness. A 71-year-old female presented with cough. Chest computed tomography (CT) revealed an irregular soft tissue mass in the right middle lobe. All autoantibodies tested were negative, and the serum IgG4 level was within the normal range. The patient showed no response to anti-infective and antitussive therapy, with follow-up CT confirming lesion enlargement. A CT-guided percutaneous lung biopsy was subsequently performed. Histopathology demonstrated infiltration of numerous lymphocytes and plasma cells, along with scattered neutrophils. Immunohistochemistry showed a ratio of IgG4-positive to IgG-positive plasma cells of approximately 42%, and demonstrated approximately 30 IgG4-positive plasma cells per high-power field (HPF). The diagnosis of IgG4-RLD was established based on the 2019 ACR/EULAR International Classification Criteria for IgG4-RD. The patient was subsequently treated with oral prednisone, which led to complete resolution of the cough and near-complete regression of the pulmonary lesion.