In immunology, an idiotype is a shared characteristic between a group of immunoglobulin or T cell receptor (TCR) molecules based upon the antigen binding specificity and therefore structure of their variable region. The variable region of antigen receptors of T cells (TCRs) and B cells (immunoglobulins) contains a complementarity determining region (CDR) with a unique amino acid structure that determines the antigen specificity of the receptor. The structure formed by the CDR is known as the idiotope. Immunoglobulins or TCRs with a shared idiotope are the same idiotype
The term idiotype is sometimes used to describe the collection of multiple idiotopes, and therefore overall antigen binding capacity, possessed by an antibody.