[Pathophysiology and Assessment of IgG4-Related Disease--Focus on Autoimmune Pancreatitis]

IgG4-related disease is well-known, and while the functions of cytokines which affect IgG4 production are being clarified, it remains unclear what causes it. There are many clinicopathological characteristics of IgG4-related disease and, therefore, comprehensive criteria are used for diagnosis. Notably, histopathological findings are the most important of these, with which we cannot make a definite diagnosis. The model disease of an IgG4-related disease is autoimmune pancreatitis (AIP). Currently, AIP is classified into type 1 (AIP1) and type 2 (AIP2). AIP1 is IgG4-related while AIP2 is not. AIP1 sometimes has localized mass formation, making it difficult to distinguish between AIP1 and pancreatic cancer. Thus, upon biochemical and immunological examination, the IgG4 level is the most useful for the diagnosis, although the levels of IL-2R, β2MG, C4, and monoclonal rheumatoid factor are also useful for the assessment of disease. In addition, histopathological findings are also important to diagnose AIP1. Typical AIP1 cases show lymphoplasmacytic infiltration including IgG4-positive plasma cells with storiform fibrosis. A careful analysis of cases with the typical features of IgG4-related disease will lead to the elucidation of the mechanism behind IgG4-related disease.