A region on the surface of an antibody molecule that contributes to the antigen-binding site (see immunoglobulin). Each of the two arms of the Y-shaped antibody comprises a heavy chain and a light chain, both of which carry three hypervariable loops. These are stretches of the polypeptide along which the amino acid sequence can vary greatly in different B-cell clones, thereby generating differently shaped antigen-binding sites, and hence antibodies with different binding specificities. The three-dimensional arrangement of each loop constitutes a CDR.