Autoimmune pancreatitis (AIP) currently encompasses two conditions: type 1 AIP, which is the pancreatic manifestation of a multiorgan IgG4-related disease, and type 2 AIP, which is limited to the pancreas. The two types share common histologic features such as periductal lymphoplasmacytic inflammation, loss of pancreatic parenchyma, and increased fibrosis, but there are morphologic differences which often allow one to distinguish between the two. Type 1 AIP is more likely to have areas of storiform fibrosis and is much more likely to have lymphocytic phlebitis. There are almost always increased numbers of IgG4-positive plasma cells. Type 2 AIP is more likely to have active (neutrophil) inflammation, particularly in duct epithelium; these lesions are termed granulocytic epithelial lesions and are a very helpful diagnostic finding. Type 2 AIP seldom has increased tissue IgG4. Both types of AIP can be mistaken clinically for malignancy. The diagnosis can often be made on large-caliber trucut biopsy.
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