Clinical features of autoimmune pancreatitis.

Autoimmune pancreatitis is a unique form of chronic pancreatitis characterized by a high serum IgG4 concentration, and complications involving various extrapancreatic lesions. It should be emphasized that autoimmune pancreatitis is easily misdiagnosed as pancreatic cancer. This disease predominantly affects elderly men. The major symptom at onset is obstructive jaundice, while severe abdominal pain is rare. Blood tests have shown abnormal results, which could be attributed mainly to the obstructive jaundice. With regard to autoantibodies, the positive rate for the antinuclear antibody is 40%; however, disease-specific autoantibodies are rarely found. About half of the patients with autoimmune pancreatitis have shown exocrine and endocrine dysfunctions. IgG4 is a sensitive and specific marker for diagnosing autoimmune pancreatitis. In differentiating between pancreatic cancer and autoimmune pancreatitis, IgG4 shows a sensitivity of 90%, a specificity of 98%, and an accuracy of 95%. HLA DRB1*0405-DQB1*0401 alleles are significantly associated with autoimmune pancreatitis, and may present a specific peptide which triggers the autoimmune response. Various imaging findings have shown pancreatic swelling, irregular narrowing of the main pancreatic duct, and stenosis of the lower bile duct. Histology shows a significant presence of lymphocytes, plasma cell infiltration, and fibrosis with abundant IgG4-bearing plasma cell infiltration. After corticosteroid treatment, imaging findings and pancreatic functions usually improve significantly. It is probable that autoimmune pancreatitis is an essentially progressive condition resulting in pancreatic stone formation in a similar way to ordinary chronic pancreatitis.