Type 2 Autoimmune Pancreatitis

Histopathology of type 2 AIP The pancreas of patients with type 2 AIP is often only focally involved. The region that seems to be most often affected is the pancreatic head including the pancreatic portion of the distal bile duct. As in type 1 AIP, the outstanding histologic feature is a periductal lymphoplasmacytic infiltrate usually affecting some or all of the medium sized ducts (Figure 1). It is often accompanied by a collar-like periductal fibrosis, with narrowing of the affected duct. The lymphoplasmacytic infiltrate may extend from the periductal area to the acinar tissue. In addition, there is a perilobular fibrosis, occasionally of the storiform-type. These histological changes can also be found in type 1 AIP, but are usually less pronounced in type 2 AIP. Conversely, the finding of the so called granulocytic epithelial lesion (GEL) is specific to AIP 2 (19). This lesion is characterized by focal disruption and destruction of the duct epithelium due to the invasion of neutrophilic granulocytes. GELs affect medium sized and small ducts (Figure 2), and may also be recognized in the acinar tissue. In the ducts they often cause destruction and obliteration of the duct lumen. The number of GELs and their severity differs from patient to patient. If a GEL is included in a biopsy specimen from the pancreas it is diagnostic for type 2 AIP (4). Another, though less specific, criterion for the diagnosis of type 2 AIP is the absence or scant (<10 cells/HPF) IgG4-positive staining plasma cells in the inflamed pancreatic tissue (Figure 3) (4, 20).