Naturally synthesized and secreted by plasma B cells, the immunoglobulin G (IgG) antibody is the most abundant antibody isotype found in blood and extracellular fluid. There are four IgG subclasses (IgG1, IgG2, IgG3, and IgG4) in humans, of which IgG1 is the most abundantnamed in serum. IgG antibodies are large molecules of about 150 kDa composed of four peptide chains, two identical heavy chains of about 50 kDa and two identical light chains of about 25 kDa. The four peptides are arranged in a Y-shape tetramer by disulfide bonds formed between the two heavy chains and between a heavy chain and a light chain.The arms of the Y, also called the Fab (fragment, antigen-binding) region containing an identical antigen binding site, is composed of one variable (located at amino terminal end, VH and VL) and one constant domain (CH1 and CL) from each heavy and light chain of the antibody. VH and VL, also called the FV region, is the most important region for binding to antigens. Three variable loops of β-strands, also called the complementarity determining regions (CDRs or idiotypes), on VH and VL respecetively are responsible for binding to the antigen.The base of the Y, also called the Fc (Fragment, crystallizable) region and composed of two heavy chains with two or three constant domains depending on the class of the antibody, binds to a specific class of Fc receptors and other immune molecules, such as complement proteins, to induce an appropriate immune response for a given antigen. The Fc region bears two highly conserved N-glycosylation sites, one on each heavy chain. The N-glycans attached to this site are predominantly core-fucosylated diantennary structures of the complex type. In addition, small amounts of these N-glycans also bear bisecting GlcNAc and α-2, 6-linked sialic acid residues.
