Immunoglobulin G4-related disease (IgG4-RD) is a systemic fibroinflammatory disorder that has been recognized to involve virtually any organ in the body and typically manifests mass-like lesions (tumefactive). Although initial reports of this disease (autoimmune pancreatitis [AIP]) were described in the Japanese population, it has since been reported worldwide. It is most commonly seen in adults of middle age or older, more often men than women. The pathogenesis of IgG4-RD is largely unknown, but genetic factors, microorganisms, and autoimmunity are thought to play important roles. Serum IgG4 concentration is elevated in the majority of patients with IgG4-RD but is a nonspecific finding. Characteristic histopathologic features include dense lymphoplasmacytic infiltrate, fibrosis (often in storiform pattern), and obliterative phlebitis. Lung involvement in IgG4-RD was first reported in 2004 in two patients with AIP and coexisting interstitial lung disease. Since then, a wide spectrum of intrathoracic involvement has been reported and includes not only parenchymal lung diseases but also pleural, airway, vascular, and mediastinal lesions. Thoracic involvement in IgG4-RD is often found incidentally during the workup of extrathoracic lesions but can sometimes be the presenting abnormality. The diagnosis of IgG4-RD requires correlation of clinical, laboratory, imaging, and histopathologic features. Glucocorticoids are the first-line therapy but other options including B cell depletion are being investigated. IgG4-RD is generally associated with an indolent clinical course and most patients improve with glucocorticoid therapy.
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