Type 1 autoimmune pancreatitis with markedly low serum IgG4 and no detectable IgG4-positive plasma cells in pancreatic tissue.

Autoimmune pancreatitis (AIP) is typically associated with elevated serum IgG4 levels and IgG4-positive plasma cell infiltration in affected organs. We report a 57-year-old woman who presented with obstructive jaundice. Contrast-enhanced computed tomography showed diffuse pancreatic enlargement, distal bile duct stricture, and retroperitoneal fibrosis. Serum IgG4 was markedly decreased and remained below the reference range on repeated measurement, while immunohistochemical staining of pancreatic tissue showed no detectable IgG4-positive plasma cells. Histology revealed storiform-pattern fibrosis with lymphoplasmacytic infiltration. Type 2 AIP was considered unlikely based on the absence of granulocytic epithelial lesions on histology and the presence of extrapancreatic involvement. Oral prednisolone resulted in rapid clinical and radiological improvement. This case highlights that type 1 AIP can occur despite markedly low serum IgG4 levels and the absence of detectable IgG4-positive plasma cells in pancreatic tissue, emphasizing the importance of a comprehensive diagnostic approach including imaging, histology, extrapancreatic manifestations, and steroid responsiveness.