By definition an immune system is specific to a single antigen which creates it, however, many naturally occurring 'antigens' are a mixture of macromolecules (eg bacteria, toxins, proteins, pollen, fungi, viruses, etc) which contain several epitopes. Contact with a complex antigen such as a virus will stimulate multiple immune responses to the different individual macromolecules that make up the virus as well as the individual epitopes of each macromolecule. For example, the tetanus toxin is a single protein macromolecular antigen but will stimulate many immune responses due to the tertiary structure of the protein acting as various epitopes for various immune responses.
Medicinal uses for this idea include immunisation to bacterial infections. The toxin that creates the immune response will have an epitope on it that stimulates the response. Denaturing of the protein may 'disarm' its function but allow the immune system to have an immune response thus creating an immunity without harming the patient.