An antibody is Y-shaped and has two identical binding sites for its antigen, one on each arm of the Y. (A) Schematic drawing of a typical antibody molecule. The protein is composed of four polypeptide chains (two identical heavy chains and two identical and smaller light chains) held together by disulfide bonds. Each chain is made up of several different domains, here shaded either blue or gray. The antigen-binding site is formed where a heavy-chain variable domain (V_H) and a light-chain variable domain (V_L) come close together. These are the domains that differ most in their sequence and structure in different antibodies. (B) Ribbon drawing of a single light chain showing the parts of the V_L domain most closely involved in binding to the antigen in red; these contribute half of the fingerlike loops that fold around each of the antigen molecules in (A).  Fair Use and Copyright info