 In IgG, IgA and IgD antibody isotypes, the Fc region is composed of two identical protein fragments, derived from the second and third constant domains of the antibody's two heavy chains; IgM and IgE Fc regions contain three heavy chain constant domains (CH domains 2–4) in each polypeptide chain.The Fc regions of IgGs bear a highly conserved N-glycosylation site.Glycosylation of the Fc fragment is essential for Fc receptor-mediated activity.