Complementarity-determining region

A single chain antibody fragment showing the positions of the three complementarity determining regions, CDR1, CDR2 and CDR3

A complementarity determining region (CDR) is a relatively short amino acid sequence in the shape of a flexible loop, found in the variable (V) domains of antigen receptors (e.g. immunoglobulin and T cell receptor). The CDRs of both immunoglobulin and the T cell receptor are the parts of these molecules that determine their specificity and make contact with a specific ligand. The CDRs are the most variable part of the molecule, and contribute to the diversity of these molecules, allowing the immunoglobulin and the T cell receptor to recognize a vast repertoire of antigens.

In the variable (V) domain of an antigen receptor there are three CDRs (CDR1, CDR2 and CDR3). Since the antigen receptors are typically composed of two polypeptide chains, there is a frequency of about six CDRs for each antigen receptor that can come into contact with the antigen (each heavy and light chain contains three CDRs). A single antibody molecule has two variable domains, wherefore it contains around twelve CDRs. About sixty CDRs can be found on a pentameric IgM molecule.

Since most sequence variation associated with immunoglobulins and T cell receptors are found in the CDRs, these regions are sometimes referred to as hypervariable domains. Among these, CDR3 shows the greatest variability as it is encoded by a recombination of the VJ in the case of a light chain region and VDJ in the case of heavy chain regions.

External links

• Complementarity+determining+regions at the U.S. National Library of Medicine Medical Subject Headings (MeSH)