So antibodies consist of four polypeptide subunits that basically bind together via disulfide bridges, disulfide bonds which are covalent bonds between sulfur atoms. And they bind in such a way to form a Y shaped structure as shown in the following diagram. Now, two of these polypeptides are heavy chains. They're large polypeptide chains. And the other two chains, shown in red, are the light chains. And that's because they're smaller than the heavy chain.