Immunoglobulin G (IgG) is a type of antibody composed of four peptide chains—two identical heavy chains and two identical light chains arranged in a Y-shape typical of antibody monomers.There are four IgG subclasses (IgG1, 2, 3, and 4) in humans, named in order of their abundance in serum. The IgG2a isotype was able to interact very efficiently with FcgammaR, and share approximately 64-78% amino acid sequence identity with IgG1 and IgG2b in the heavy chain constant region domains, CH1, CH2 and CH3, which cause the diffrence in the interaction with complement, NK and mast cells, induction or inhibition by specific cytokines, and ability to cross the placenta.