IgG4 Subclass of Immunoglobulins; Immunobiology and Roles in Relation to Human Diseases

IgG4, a subclass of antibodies known as immunoglobulins have unique structural features, in particular, their Fc regions, that prevents their interactions with other receptors on effector cells and thus disabling them of activating complements system. IgG4 antibodies can undergo a process called Fab-arm exchange, wherein they exchange half-molecules with other IgG4 antibodies, thus forming bispecific monovalent antibodies. Isotypic switch in mature B cells in germinal centres of secondary lymphoid organs is controlled by Tfh subset of T cells. Functionally IgG4 antibodies exert immunomodulatory and blocking activities, modulating protective inflammation evolved by parasitic invasion and allergic inflammation. From the pathophysiological point of view, IgG4 autoantibodies are prominently observed in autoimmune diseases under the umbrella of IgG4-autoimmune diseases (IgG4-AID). Furthermore, IgG4-related diseases (IgG4-RD) are affecting various organs characterized by lymphoplasmacytic infiltrates and storiform fibrosis in tissues, together with elevated IgG4 levels in the blood. A better understanding of IgG4 immunobiology helps us diagnose and treat patients suffering from these rare forms of diseases.