Recent studies have suggested the existence of two subtypes of
autoimmune pancreatitis (AIP):
type 1 AIP, related to
IgG4 (lymphoplasmacytic sclerosing
pancreatitis); and
type 2 AIP, related to a granulocytic epithelial lesion (idiopathic duct-centric
chronic pancreatitis). Compared with
type 2 AIP, the clinicopathological features of
type 1 AIP, with increased serum
IgG4/
IgE levels, abundant infiltration of
IgG4 + plasmacytes and lymphocytes,
autoantibodies, and
steroid responsiveness, are more suggestive of abnormal immunity such as
allergy or autoimmunity. Moreover, patients with
type 1 AIP often have extrapancreatic lesions, such as
sclerosing cholangitis, sclerosing
sialadenitis, or
retroperitoneal fibrosis, showing pathological features similar to those of the pancreatic lesions. Based on these findings, an international concept of and diagnostic criteria for AIP have been proposed recently. Of interest, many synonyms have been proposed for the conditions of AIP and extrapancreatic lesions associated with
IgG4, such as "multifocal idiopathic fibrosclerosis," "
IgG4-related
autoimmune disease," "
IgG4-related sclerosing disease," "systemic IgG4-related plasmacytic syndrome (SIPS)," and "
IgG4-related multiorgan lymphoproliferative syndrome," all of which may refer to the same conditions. Therefore, the Japanese Research Committee for "Systemic
IgG4-Related Sclerosing Disease" proposed a disease concept and clinical diagnostic criteria based on the concept of multifocal fibrosclerosing disease, in 2009, in which the term "
IgG4-related disease" was agreed upon as a minimal consensus to cover these conditions. Although the significance of
IgG4 in the development of "
IgG4-related disease" remains unclear, we have proposed a hypothesis for the development of
type 1 AIP, one of the
IgG4-related diseases. The concept and diagnostic criteria of "
IgG4-related disease" will be changed in accordance with future studies.