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. So we have these two light chains that are bonded via disulfide bridges to these two heavy chains. And the two heavy chains are also bonded to each other by these disulfide bonds. Now, on every single antibody, we have a region known as the constant region and we have a region known as the variable region.