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. So let's suppose we have the antigens floating around in our blood or in our lymph or in our tissue. Now, when this specific antibody that was built by the plasma cell approaches these specific antigens for which it was built for in the first place, we have a binding process taking place and the epitome basically binds onto the antigen binding side of this antibody as shown in the following diagram. And then we form our antibody antigen complex which then elicits some type of defense or response as we'll discuss in just a moment.