Antibodies are macromolecules produced by the cells of our body and the cells of other organisms and they serve a protective function. So they are part of the immune system of that particular organism. So whenever pathogenic agents some type of infectious agent, for example, a bacterial cell makes its way into the organism, that organism responds by using its immune immune system to basically produce these special molecules we call antibodies. 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.