<P> 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 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 α - 2, 6 linked sialic acid residues . </P> <P> The other part of an antibody, called the Fab region, contains variable sections that define the specific target that the antibody can bind . By contrast, the Fc region of all antibodies in a class are the same for each species; they are constant rather than variable . The Fc region is, therefore, sometimes incorrectly termed the "fragment constant region". </P>

The fc portion of an antibody is formed by
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