IgG4-related disease: a systemic condition with characteristic microscopic features.

During the first decade of the 21st century, IgG4-related disease (IgG4-RD), a fibroinflammatory condition occurring at multiple sites of the body, has been newly recognized. As indicated by its name, elevation of IgG4 in the serum and tissue is a common denominator of IgG4-RD. Since the observation that many patients suffering from autoimmune pancreatitis (AIP), a specific type of chronic pancreatitis, had elevated serum levels of IgG4, it was reported that these patients also had increased numbers of IgG4-positive cells in the inflamed pancreatic tissue. In 2003, it was noted that a significant proportion of the AIP patients had a variety of extrapancreatic fibroinflammatory lesions, and that AIP therefore was the pancreatic manifestation of a systemic disease. Among these extrapancreatic manifestations, the extrahepatic bile ducts, salivary glands, thyroid, lymph nodes and retroperitoneum were most frequently reported, and infiltration of the tissue with IgG4-positive cells was also noted at these sites. During the following years, a multitude of other conditions have been added to the spectrum of IgG4-RD. While some of these organ manifestations were once believed to represent diseases on their own, others have been included under the umbrella of "multifocal fibrosclerosis". Biopsies or resection specimens from affected organs in IgG4-RD reveal several common microscopic features irrespective of the site of the lesion. Cellular and storiform fibrosis, lymphoplasmacytic infiltration, increased numbers of IgG4-positive cells and obliterative phlebitis are among the most characteristic histological changes in IgG4-RD. The detailed etiology, pathophysiology, epidemiology and clinical long-term outcome have at present yet to be fully elucidated. This paper focuses on the microscopic features, diagnosis and differential diagnosis of the different organ manifestations of IgG4-RD, and the current concepts of its pathogenesis will also be addressed.