An idiotype is characterized by

Idiotypes are antigenic determinants that appear only on the Fab fragments of antibodies and appear to be localized at the ligand-binding site; thus, anti-idiotype antisera may block reactions with the appropriate hapten. The carbohydrate side chains of immunoglobulins are relatively nonimmunogenic. New determinants may be exposed after papain cleavage of immunoglobulins, but these determinants are not included in the classification of the native molecule. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition