A typical antibody monomer has four protein chains, ie, two indentical light $(L)$ chains and two identical heavy $(H)$ chains. The chains are joined by disulphide links and other bonds to form 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 $(H)$ form one antigen binding site. Hence, each antibody has two antigen binding sites.