Another important function of Ii is to accelerate the binding of epitopes of exogenously derived antigens to MHC class II molecules. Antigen Express scientists discovered that a portion of the Ii protein interacts with an allosteric site on MHC class II molecules. The interaction of this portion of the molecule, termed Ii-Key, with the allosteric site loosens the hold of the antigen binding trough on any resident epitope. Hybrid peptides containing the Ii-Key ligand thereby cause the discharge of any epitope resident in the MHC Class II molecule, allowing the peptide epitope linked to Ii-Key to occupy the antigen binding trough.  The result is much more efficient MHC class II presentation of any epitope linked to Ii-Key compared to an epitope-only peptide, as illustrated in the figure.  By these means, Ii-Key hybrid peptides bypass all requirements for intra-cellular antigen processing and directly 'hijack' MHC class II molecules already on the surface of cells to present whatever epitope is present in the hybrid. The result is a much more robust and long lasting epitope-specific immune response.

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