Antibody structure. (A) Immunoglobulin G (IgG) consists of four chains, two heavy chains (blue) and two light chains (red), linked by disulfide bonds. The heavy and light chains come together to form F_ab domains, which have the antigen-binding sites at the ends. The two heavy chains form the F_c domain. Notice that the F_ab domains are linked to the F_c domain by flexible linkers. (B) A more schematic representation of an IgG molecule.

[Drawn from 1IGT.pdb.]