In a mouse model of autoantibody mediated anemia using IgG isotype switch variants of an anti erythrocytes autoantibody, it was found that mouse IgG2a was superior to IgG1 in activating complement. The relative ability of different IgG subclasses to fix complement may explain why some anti-donor antibody responses do harm a graft after organ transplantation. The relative balance of these subclasses, in any immune complexes that form, helps determine the strength of the inflammatory processes that follow.įinally, if antigen persists, high affinity IgG4 is produced, which dampens down inflammation by helping to curtail FcR-mediated processes. Subsequently, higher affinity IgG1 and IgG2 are produced. The IgG3, though of relatively low affinity, allows IgG-mediated defences to join IgM-mediated defences in clearing foreign antigens. Given the opposing properties of the IgG subclasses fixing and failing to fix complement binding and failing to bind FcRand the fact that the immune response to most antigens includes a mix of all four subclasses, it has been difficult to understand how IgG subclasses can work together to provide protective immunity. The structure of the hinge regions region 6 in the diagram contributes to the unique biological properties of each of the four IgG classes.
Note: IgG affinity to Fc receptors on phagocytic cells is specific to individual species from which the antibody comes as well as the class. There are four IgG subclasses IgG1, 2, 3, and 4 in humans, named in order of their abundance in serum IgG1 being the most abundant. The Fc regions of IgGs bear a highly conserved N-glycosylation site at asparagine in the constant region of the heavy chain. The various regions and domains of a typical IgG are depicted in the figure to the left. Each end of the fork contains an identical antigen binding site. The resulting tetramer has two identical halves, which together form the Y-like shape. IgG antibodies can prevent IgE mediated anaphylaxis by intercepting a specific antigen before it binds to mast cell-associated IgE.Ĭonsequently, IgG antibodies block systemic anaphylaxis induced by small quantities of antigen but can mediate systemic anaphylaxis induced by larger quantities. IgG are also involved in the regulation of allergic reactions.
This repertoire of immunoglobulins is crucial for the newborns who are very sensitive to infections, especially within the respiratory and digestive systems. Therefore, in the first six months of life, the newborn has the same antibodies as the mother and the child can defend itself against all the pathogens that the mother encountered in her life even if only through vaccination until these antibodies are degraded. In individuals with prior immunity to a pathogen, IgG appears about 24-48 hours after antigenic stimulation. Colostrum contains a high percentage of IgG, especially bovine colostrum. Along with IgA secreted in the breast milkresidual IgG absorbed through the placenta provides the neonate with humoral immunity before its own immune system develops. It is the only antibody isotype that has receptors to facilitate passage through the human placentathereby providing protection to the fetus in utero. IgG is secreted as a monomer that is small in size allowing it to easily perfuse tissues.
IgG antibodies are generated following class switching and maturation of the antibody response, thus they participate predominantly in the secondary immune response.
Portal 2 Download Freeīy binding many kinds of pathogens such as virusesbacteriaand fungiIgG protects the body from infection. IgG is the main type of antibody found in blood and extracellular fluidallowing it to control infection of body tissues. Antibodies are major components of humoral immunity. Immunoglobulin G IgG is a type of antibody.