Autoimmune pancreatitis (AIP) is a relatively rare disease that has recently emerged as a unique type of pancreatitis with a presumed autoimmune etiology. Sarles et al.  first reported a form of idiopathic chronic pancreatitis which might be caused by an autoimmune mechanism in 1961. It is uncertain whether the pancreatitis they reported coincides with AIP as we currently understand it. In 1991, Kawaguchi et al.  reported two cases of unusual inflammatory disease, involving the pancreas and biliary tract that were resected on suspicion of pancreatic cancer, and described the histology as lymphoplasmacytic sclerosing pancreatitis (LPSP). In 1992, Toki et al.  reported four cases with an unusual type of chronic pancreatitis showing diffuse, irregular narrowing of the entire main pancreatic duct (MPD) on endoscopic retrograde pancreatography (ERP) scans. This type of pancreatitis was described in the marginal notes of the newly established the JPS criteria for chronic pancreatitis in 1995 as “duct-narrowing chronic pancreatitis,” a chronic inflammation of the pancreas which may not fulfill the criteria of chronic pancreatitis . In 1995, Yoshida et al.  proposed AIP as a diagnostic entity. They summarized the clinical features as follows: increased serum γ-globulin or immunoglobulin (Ig)G levels and the presence of autoantibodies; diffuse irregular narrowing of MPD and enlargement of the pancreas; occasional association with stenosis of the lower bile duct and other autoimmune diseases; mild symptoms, usually without acute attacks of pancreatitis; effectiveness of steroid therapy; and histological findings of LPSP [5, 6]. Thereafter, AIP has been extensively reported worldwide including from Japan, Korea, Europe, and the USA. In this chapter, we will describe the diagnostic criteria of AIP, mainly focusing on the International Consensus Diagnostic Criteria (ICDC)  and those proposed by the JPS [8–10].
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