IgG4-related Disease Affecting Multiple Organs Despite Normal Serum IgG4 Levels

IgG4-related disease (IgG4-RD) is characterized primarily by elevated serum IgG4 levels and it can affect multiple organs. We herein present a case of IgG4-RD affecting the kidneys, right submandibular gland, lungs, mediastinal lymph nodes, and retroperitoneum, despite normal serum IgG4 levels. In this case, a histopathological examination of the kidneys and submandibular glands revealed marked lymphoplasmacytic infiltration and a relatively small number of IgG4-positive plasma cells. The patient was diagnosed with IgG4-RD and showed a favorable response to corticosteroid therapy. This case highlights the importance of a histopathological evaluation for diagnosing IgG4-RD, particularly in patients with normal serum IgG4 levels.