Autoimmune pancreatitis: updated concepts of a challenging diagnosis.

Autoimmune pancreatitis is a benign process characterized by inflammation and fibrosis. It is now known that cases of "autoimmune pancreatitis" actually consist of two distinct pathologic entities. Type 1 autoimmune pancreatitis is a manifestation of a systemic process, immunoglobulin G subclass 4 (IgG4)-related disease. IgG4-related disease can affect virtually every organ system in the body. Type 1 affects older patients and is characterized by an elevated serum IgG4 level and sites of extrapancreatic disease. Type 2 autoimmune pancreatitis is a disease process confined to the pancreas. It affects younger patients and is associated with inflammatory bowel disease. Type 2 is not associated with elevated IgG4 levels or extrapancreatic disease. Both subtypes can mimic malignancy, particularly pancreatic cancer. Awareness of the clinical and imaging features of the subtypes of autoimmune pancreatitis is important to avoid an incorrect diagnosis of malignancy.