The
fragment crystallizable region (
Fc region) is the tail region of an
antibody that interacts with cell surface receptors called
Fc receptors and some proteins of the
complement system. This property allows antibodies to activate the
immune system. 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 (C
H 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. 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 a-2,6 linked
sialic acid residues.