Explanation

A typical antibody monomer has four protien chains, i.e., two identical light (L) chains and two identical heavy (H) chains. The chains are joined by disulphide links and other bonds to from a Y-shaped structure. Both the heavy (H) and light (L) chains have variable regions at the ends of Y arms. The variable region from one light chain (L) and one heavy chain (H)from one antigen binding site. Hence, each antibody has two antigen binding sites.