IgG4 is a subclass of immunoglobulin G (IgG), which comprises four main subclasses: IgG1, IgG2, IgG3, and IgG4. IgG4 primarily participates in immune regulation within the immune system and plays a significant role in certain inflammatory responses. For instance, it can form cross-links with other antibodies, preventing them from binding to antigens, thereby reducing the intensity of inflammatory reactions. Elevated IgG4 levels are often observed in autoimmune diseases such as systemic lupus erythematosus, rheumatoid arthritis, and Sjögren's syndrome.