IgG4 related disease

**Definition:** IgG4 related syndrome (IgG4-related disease: IgG4-RD) is a clinical disease characterised by elevated serum IgG4 concentration and tumefaction or tissue infiltration by IgG4-positive plasma cells. The diagnostic criteria for IgG4 related syndrome have been proposed, and it may be present in a certain population of patients with a wide variety of diseases, including Mikulicz disease, autoimmune pancreatitis, hypophysitis, Riedel thyroiditis, interstitial pneumonitis, interstitial nephritis, prostatitis, lymphadenopathy, retroperitoneal fibrosis, inflammatory aortic aneurysm, and inflammatory pseudo tumour.