IgG4-related sclerosing cholangitis (IgG4-SC) is histologically characterized by the inflammation observed around the bile duct epithelium with the epithelium left intact. Transmural inflammation with storiform fibrosis that gives rise to wall thickening is a highly typical histological finding, which may be solely sufficient for diagnosis. Involvement of the connective tissue surrounding the bile ducts is common, and if this involvement is eminent, it appears as a mass (inflammatory pseudotumor). An increase of IgG4-positive cells is a characteristic, but not specific histological feature. In contrast to other diseases that cause an increase of IgG4-positive cells, such as biliary cancers, primary sclerosing cholangitis, and cholelithiasis, the presence of IgG4-positive cells is diffuse, and the IgG4/IgG-positive cell ratio is high. The diagnosis of IgG4-SC with bile duct biopsies is difficult because lesions of IgG4-SC are present in the deep part of the bile duct wall. Liver biopsies may obtain peripheral portal tracts packed with lymphoplasmacytic infiltration, but characteristic bile duct lesions are absent. Thus, the appropriate clinical setting is necessary to make a diagnosis of IgG4-SC with biopsy speciments.
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