IgG4-related disease. Microscopic diagnosis and differential diagnosis

IgG4-related disease (IgG4-RD) is a potentially systemic inflammatory and fibrosing disease. Independent of the affected organ, the main histological features are a dense lymphoplasmacytic infiltrate, storiform fibrosis, and obliterative phlebitis. These features are weighted slightly different, dependent on the respective organ. If several of these features are present and changes inconsistent with IgG4-RD are absent, the diagnosis is further supported by the immunohistochemical demonstration of increased IgG4-positive plasma cells and an elevated IgG4-IgG ratio, as well as serologically by elevated IgG4 levels. A tissue biopsy is often mandatory for the diagnosis, as IgG4-RD clinically and radiologically can imitate a malignant tumor.