A healthy adaptive immune system can raise antibodies that recognize and bind with high affinity to almost any stable molecule. The molecule to which an antibody binds is known as "antigen." Antibodies have been exploited by enterprising scientists to generate valuable tools for research, diagnosis, and therapy. One clever application is the generation of antibodies that function like enzymes to catalyze complicated chemical reactions. If you wished to produce such a "catalytic" antibody, what would you suggest using as the antigen? Should it be the substrate of the reaction? The product? Something else?