What are anti-idiotype antibodies?

Anti-idiotype antibodies are directed against the binding sites of other antibodies that react with specific antigenic determinants (epitopes). Anti-idiotype antibodies may block immunological reactions as they resemble the epitope to which the first antibody reacts.

In this case study, anti-idiotype antibodies are mentioned as one of several possible hypotheses that may result in severe hemolysis when IV RhIG is used to treat ITP. Specifically, anti-idiotype antibodies in IV RhIG may form immune complexes that trigger the binding of complement and result in complement-mediated lysis of red cells.