The paratope region of an immunoglobulin (variable domine of an antibody) is a good immunogenic part in immunoglobulins. Idiotypes are the antigenic determinants created by the hypervariable regions, localized on the Fab fragment of the immunoglobulin molecules and the anti-idiotypic antibodies are those directed against the hypervariable regions of an antibody.
In many instances the actual antigenic determinant (i.e. idiotype) may include some of the framework residues near the hypervariable region. Immune responses may be regulated by anti-idiotypic antibodies directed against our own antigenic determinants. In some cases anti-idiotypic antibodies can be used to stimulate B cells to make antibodies that can act as a vaccine.
This approach is being tried to immunize against highly dangerous pathogens that cannot be safely used as a vaccine. Idiotypes are useful markers for a particular variable region.