The variable region (V region) of an antibody, the part of the molecule that binds to antigen, is itself antigenic and can elicit anti-V region antibodies when injected into animals of a different or even of the same species. The antigens of the V region constitute its idiotype, and they are defined by anti-idiotypic antibodies in polyclonal antisera or by monoclonal anti-idiotypic antibodies. An idiotype actually consists of multiple antigenic determinants, each of which is an idiotope. The antigenic determinants or idiotopes can reside in the heavy chain component of the V region, in its light chain component, or they may consist of a surface made up of parts of both chains. The idiotype of an antibody is, therefore, a way to describe by serological means the variable region of an antibody molecule. Originally, idiotypes were defined by heterologous antisera made, for example, by immunizing a rabbit with a monoclonal human myeloma protein. After extensive absorption with normal human immunoglobulins, such antisera were found to bind only to the V region of the immunizing myeloma immunoglobulin. Thus, idiotypes were originally thought to be unique markers of individual antibodies (hence their name). However, it is now known that different antibodies can share the same, or similar, idiotype. Even antibodies with different antigen-binding specificities can share the same idiotype if they use the same heavy or light chain in forming their V regions. Such antibodies constitute parallel sets. Thus, idiotypes can be: private (confined to a particular immunoglobulin molecule), public (shared by different antibodies), or cross-reactive (different idiotypes containing similar antigenic structures)
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