Immunological principles of IgG4 related diseases

In contrast to the other IgG subclasses, IgG4 does not bind to low affinity Fc receptors or activate the classical complement pathway. In addition, it is unstable and can dissociate into two hemimolecules; therefore, IgG4 most likely has an immunosuppressive role. On the other hand, there a few examples of an immunostimulatory role of IgG4 antibodies; therefore, the function of IgG4 in IgG4 related diseases is not yet entirely clear. The trigger factors of IgG4 related diseases (allergic or autoimmune) are still under debate. The activation of T helper (Th) 2 and regulatory T cells has been shown to be important in the pathophysiology of IgG4 related diseases as they produce cytokines which contribute to the formation of IgG4 and to fibrosis of various tissues.