Epitope: As related to the protein antigens, B-cell epitopes comprises the amino acid residues of the protein molecule in which interact directly through the noncovalent bonds with amino acid residues of the particular antibody molecule (complementarity determining the area). The average epitope probably includes about 15-20 contact amino acid residues, but one or two of these might be critical to the epitope's specificity and the avidity of the antibody-antigen reaction. B-cell epitopes might be either linear in nature or conformational in nature. T-cell epitopes represent small, processed peptides which bind to the MHC class I and II molecules on surface of T cells.