Identification of specificity-determining residues in antibodies.

The successful identification of the residues that contact ligand has important implications, especially in view of the increasing use of antibodies in various medical and industrial applications. Analysis of the crystallographically derived, three-dimensional structures of five antibody-antigen complexes and of the available amino acid sequence data on antibody variable regions reveals that the residues that contact antigen are in the main also the most variable. It is proposed that a good first guess of the identity of the specificity-determining residues can be made from an examination of the variability values at sequence positions. New boundaries for the complementarity-determining regions are proposed.