Autoimmune pancreatitis: a guide for the histopathologist

Autoimmune pancreatitis (AIP) is a distinct form of pancreatitis with a characteristic histological appearance. Clinically and radiologically, many of these patients show enlargement of pancreas and pancreatic duct/bile duct strictures, thus mimicking pancreatic carcinoma. There are 2 forms of the disease: (1) type 1 AIP characterized by storiform type fibrosis, obliterative phlebitis, and elevated numbers of immunoglobulin G4 (IgG4) positive plasma cells, typically >50 per high-power field, and, (2) type 2 AIP characterized by granulocytic epithelial lesions and only occasional IgG4-bearing plasma cells, typically <10 per high-power field. The type 1 variant of AIP is the pancreatic manifestation of IgG4-related disease, thus both pancreatic and extrapancreatic recurrences are common. The type 2 variant is unrelated to IgG4-related disease, and disease recurrence is uncommon. Both forms of the disease show a swift response to immunosuppressive therapy. This review highlights the clinical and pathological differences between the 2 forms of AIP. We also review guidelines that assist in distinguishing AIP from its closest mimic, pancreatic adenocarcinoma.