And what an antibody is it's basically this very highly specialized type of protein that has a high affinity for a specific type of pathogenic antigen. Now, what exactly is an antigen? Well, an antigen is any type of molecule that comes from that infectious agent that invaded that body. 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.