Autoimmune pancreatitis (AIP) is a distinct disease entity that has been characterized clinically, serologically, radiologically and pathologically. These characteristic features were first reported by Japanese authors. However, histopathological reports on Japanese AIP patients are rare. Histopathologically, AIP in Japanese patients is defined as lymphoplasmacytic sclerosing pancreatitis (LPSP), with the following characteristic features: (a) dense circumferential lymphoplasmacytic infiltration within and around the pancreatic ducts, especially the medium-sized, interlobular and main pancreatic ducts, and (b) prominent sclerotic collagen bundles with lymphoplasmacytic infiltration. Plasmacytes show various degrees of positivity for IgG4 on immunohistochemistry.
Copyright © 2007 S. Karger AG, Basel
From a historical perspective, a number of cases reported by Japanese researchers have been very interesting contributions to understanding the histopathology and pathophysiology of conditions or diseases related, or suggested to be related, to autoimmune pancreatitis (AIP). Examples are (a) the first attempt to administer corticosteroids and the first demonstration of the effectiveness of corticosteroids in a patient with 'abdominal mass (complicated by) with Sjögren syndrome', which is currently called AIP ; (b) the first detailed description of lymphoplasmacytic sclerosing pancreatitis (LPSP) with cholangitis ; (c) the first presentation of characteristic findings of narrowing of the main pancreatic duct (MPD) on endoscopic retrograde pancreatography (ERP)  and the proposal of the concept of AIP , and (d) the first description of a high serum level of IgG4 in patients with AIP .
Thus, a small number of histopathological findings of so-called AIP were presented in Japan, and then compared to those of AIP in other countries. There have been few detailed histopathological reports on Japanese AIP .
Fig. 1. Gross serial cut surfaces demonstrated whitish-grey mass with relatively clear margins at the interface with non-affected parenchyma. The main pancreatic duct (arrow) was stenotic or narrowed, but remained identifiable throughout the mass lesion.
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