So an antigen could be, for example, a sugar molecule, a polysaccharide that came from that infectious agent. It can also be a nucleic acid or it can be a protein. And in this lecture we're going to assume our antigen is a protein. Now how exactly does the binding process between an antibody and an antigen actually take place? Well, in our body we have five categories of antibodies and antibodies are also known as immunoglobulins. And that's because immuno means it's part of our immune system and globulin means the spherical shape, the globelike shape of that particular antibody.