Remember, proteins are sequences of amino acids and the variable regions is billed to contain a specific sequence of amino acids that can bind to that specific antigen for which it was actually built for. And that's exactly why these regions vary when we go from one antibody to a different antibody because we have to change the sequence of amino acids for them to actually bind onto different types of antigens. Now, on top of that, it's these variable regions that contain these clefts, that contain these regions that actually bind onto our antigen. So these are known as the antigen binding sites. Now, the region of the antigen that the antibody is actually built for, that binds onto the antigen binding side of the antibody is known as the antigenic determinant or simply the epitope. So if we look at the following diagram, we have our antibody that contains the constant region and it contains the variable region and it's the variable region that contains the antigenic binding sites, as shown, that can actually bind onto the epitope of our antigens.