Now, if we take a look at the following diagram of the antibody, these boxed in regions are the variable regions and the rest is the constant region. Now, what exactly is the variable region? Well, as the name applies, these variable regions actually contain specific types of sequences that can bind to that antigen. 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.