[Usefulness of measuring serum IgG4 level as diagnostic and treatment marker in IgG4-related disease].

IgG4-related disease is a chronic disorder, which is characterized with elevated levels of serum immunoglobulin (Ig)G4 and abundant infiltration of IgG4-positive plasmacyte and storiform fibrosis in the enlarged organs. It includes Mikulicz's disease (IgG4-related dacryoadenitis and sialadenitis), autoimmune pancreatitis type I, and so on. In Japan, we have been able to measure the IgG4 levels in our clinic since 2010, and we knew that various diseases except IgG4-related disease, also presented with elevated levels of serum IgG4. Eosinopihic disorders, such as Chrug-Strauss syndrome, a part of rheumatoid arthritis and systemic sclerosis can present with high levels of serum IgG4. So the confusion is seen in some clinicians, but we have to recognize that only serological findings cannot lead to the correct diagnosis in IgG4-related disease. The pathological and image findings also are needed. With regard of the treatments for IgG4-related disease, the levels of serum IgG4 often reflected with the therapeutic response, and were also the markers, which predicted the relapse. We describe herein the significance of measuring serum IgG4 levels in the diagnosis and during the treatment for IgG4-related disease.