The flaws that mar proteins as drugs would be a lot easier to eliminate, or at least reduce, were it not for the one thing that gives protein engineers fits: allergic reactions. The protein engineer's doctoring arts are balm for many a malady, but not allergic reactions. A protein too unstable, too toxic, maybe too costly to manufacture, or burdened by some other problem, changes for the better when the appropriate amino acid residues are altered. The catch is that protein engineers never know when replacing one residue with another will make a protein immunogenic, provoking allergic reactions in people exposed to it. The same catch applies, of course, with proteins intended for personal care products. Agricultural and industrial proteins have the same problem.

The trouble arises when a new amino acid, together with surrounding residues, creates an allergenic peptide --a T-cell epitope--capable of triggering an immune response. Without a...

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