Immunoglobulin G4 (IgG4), one of the four human IgG subtypes, is a monomer immunoglobulin mainly involved in secondary antibody reaction, which is produced and secreted by B cells. The IgG tetramer consists of two heavy chains (50 kDa) and two light chains (25 kDa) connected by disulfide bonds, which are two identical halves to form a Y shape. After pepsin cleavage, IgG was divided into two F (ab) s with one antigen binding site and a highly conserved Fc segment. The Fc segment has a highly conserved n-glycosylation site. IgG4 is the least common IgG among healthy adults, accounting for about 5% of the total IgG library. Although the amino acid sequence of IgG4 is about 90% homologous to other IgG subclasses, IgG4 is unique because it is a univalent function with little or no inflammation, and IgG4 Fc can bind to other Fc from all human IgG subclasses. IgG4 is generally considered to be a protective blocking antibody because it can inhibit or prevent inflammation by binding to inflammatory IgG subclasses or IgE competitive antigens. In addition, IgG4 can cause a subset of autoimmune diseases in severe diseases.
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