An antigen binding site is a reflection of the three-dimensional structure of the antigen. This unique amino acid structure in the antibody is known as the idiotype. Antibodies (anti-idiotypes) can be raised against the  idiotype  in the antibody by injecting the antibody into another animal. This gives us an anti-idiotype antibody and this, therefore, mimics part of the three dimensional structure of the antigen. This can be used as a vaccine. When the anti-idiotype antibody is injected into a vaccinee, antibodies (anti-anti-idiotype antiobodies) are formed that recognize a structure similar to part of the virus and might potentially neutralize the virus. This happens: Anti-ids raised against antibodies to hepatitis B S antigen elicit anti-viral antibodies.