= Fragment crystallizable region
{wiki}
= Fc fragment
{c}
{synonym}
The <Fc fragment> comprises paired <immunoglobulin heavy chain> $C_{H2},C_{H3}$ portions, interacts with immune partners and lacks <paratopes>. Its name records historical crystallization.<Immunobiology: The Immune System in Health and Disease, 5th edition by Charles A. Janeway, Jr. and others/The structure of a typical antibody molecule>[Immunobiology: The Immune System in Health and Disease, 5th edition by Charles A. Janeway, Jr. and others; §3-3: crystallization-based name, paired CH2 and CH3, and effector interactions]{ref}
For example, <Fc fragments> can bind components of the <complement system>, illustrating a function distinct from <epitope> recognition.<Antibody Structure and Molecular Immunology by Gerald M. Edelman>[Antibody Structure and Molecular Immunology, printed p. 37: Fc binding to complement components]{ref}
An isolated <Fc fragment> therefore separates one set of <protein> interactions from target recognition. In the connectivity example under <Papain digestion of IgG>, the <Fc fragment> contributes one connected product but zero <antibody binding sites>. Retaining it preserves different interaction surfaces; it does not restore either severed arm. Conversely, the two <Fab fragments> preserve the stipulated <epitope>-recognition sites while losing their connection to this common stem.
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