Clinical Information

The most abundant immunoglobulin isotype in human serum is IgG. IgG immunoglobulins are comprised of 4 subclasses, designated IgG1 through IgG4. Of total IgG, approximately 65% is IgG1, 25% is IgG2, 6% is IgG3, and 4% is IgG4. Each IgG subclass contains structurally unique portions of the constant region of the gamma heavy chain.

IgG subclass 4-related disease is a  systemic inflammatory disease of unknown etiology, most often occurring in middle-aged and older men. Several organ systems can be involved, and the disease encompasses many previous and newly described diseases such as autoimmune pancreatitis; Mikulicz disease and sclerosing sialadenitis; inflammatory orbital pseudotumor; chronic sclerosing aortitis; Riedel thyroiditis, a subset of Hashimoto thyroiditis; IgG4-related interstitial pneumonitis; and IgG4-related tubulointerstitial nephritis. These entities may be characterized by tumor-like swelling of the involved organs with infiltration by numerous IgG4-positive plasma cells with accompanying fibrosis. In addition, elevated serum concentrations of IgG4 are found in at least 50% of patients diagnosed with IgG4-related disease.

The diagnosis of IgG4-related disease may require a tissue biopsy of the affected organ demonstrating the aforementioned histological features. It is recommended that patients suspected of having an IgG4-related disease have their serum IgG4 measured.