Proteins, by their very nature, are antigens. A human protein, innocuous at its typical physiological concentration, may exhibit completely different immunogenic properties when administered in the higher concentration that would be used as a drug. Unless a biotechnology-derived protein is engineered to be 100% complementary to the human form, it will differ among several major epitopes. The protein may have modifications of its amino acid sequence (substitutions of one amino acid for another). There may be additions or deletions of amino acids, N-terminal methionyl groups, incorrect or abnormal folding patterns, or oxidation of a sulfur-containing side chain of a methionine or a cysteine. Additionally, when a protein has been produced by using a bacterial vector, a finite amount of immunoreactive material may pass into the final product. All of these listed items contribute to the anti-genicity of a biotechnologically produced protein. When it is administered to a human patient, the host's immune system will react to the protein just as it would to a microbial attack and neutralize it. This is why research has been undertaken to create 100% human protein drugs, such as insulin, which patients will need to take for a long time. In addition, some of the most promising biotechnology products, the MAbs, are produced in mice by use of humanized genes to avoid human reaction to the mouse antibody.
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