The region of an antigen that interacts with an antibody is defined as an epitope. For protein antigens, epitopes may involve a single length of the polypeptide chain (sequential or linear epitopes) or may be composed of several widely separated, discrete amino acid sequences that come together in the folded native portion (conformational or discontinuous epitopes) (1 ). Complete definition of the structure of an epitope can be achieved by X-ray crystallography of antigen-antibody cocrystals, but to date only a limited number of protein epitopes (all of the discontinuous type) have been defined by this method (1 ,2 ). These studies, however, have suggested that the epitopes of native protein consist of 15–22 residues with a smaller subset of 5–6 residues contributing most of the binding energy. It is important to note that these critical residues may not be arranged in a linear sequence (1 ).
