Autoimmune pancreatitis type 1

Autoimmune pancreatitis type 1 (AIP type 1), also known as lymphoplasmacytic sclerosing pancreatitis or IgG4-related pancreatitis, is a distinct form of chronic pancreatitis that occurs as part of a systemic IgG4-related disease (IgG4-RD). It is characterized by elevated serum IgG4 levels and dense infiltration of IgG4-positive plasma cells into the pancreas and other organs. The disease predominantly affects the pancreas, causing diffuse or focal enlargement that can mimic pancreatic cancer on imaging. Patients commonly present with obstructive jaundice, abdominal pain, weight loss, and new-onset diabetes mellitus. Because AIP type 1 is a manifestation of systemic IgG4-related disease, extrapancreatic involvement is frequent and may include the bile ducts (IgG4-related sclerosing cholangitis), salivary glands, retroperitoneum, kidneys, lymph nodes, and other organs. The disease predominantly affects middle-aged to elderly males, with a male-to-female ratio of approximately 3:1. Diagnosis is based on a combination of imaging findings (diffuse pancreatic enlargement with delayed enhancement, a capsule-like rim), serology (elevated serum IgG4), histopathology (lymphoplasmacytic infiltrate, storiform fibrosis, obliterative phlebitis), extrapancreatic organ involvement, and response to corticosteroid therapy. Several diagnostic criteria systems exist, including the International Consensus Diagnostic Criteria (ICDC) and the HISORt criteria. The cornerstone of treatment is corticosteroid therapy, to which AIP type 1 typically shows a dramatic response. Initial treatment usually involves oral prednisolone, with gradual tapering over several weeks to months. However, relapse rates are significant, occurring in 30–50% of patients, particularly those with extrapancreatic involvement or persistently elevated IgG4 levels. For relapsing or refractory cases, steroid-sparing immunosuppressive agents such as azathioprine, mycophenolate mofetil, or rituximab (an anti-CD20 monoclonal antibody) may be employed. Long-term monitoring is essential to detect relapses and manage complications such as pancreatic exocrine insufficiency and diabetes.

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