Three regions (CDR1; CDR2 and CDR3) of amino acid sequence in the immunoglobulin variable region that are highly divergent. Together the CDRs from the light and heavy immunoglobulin chains form a surface that is complementary to the antigen. These regions are also present in other members of the immunoglobulin superfamily, for example, T-cell receptors (receptors, antigen, t-cell).