One important application of recombinant DNA technology is the mass production of proteins. Insulin, for example, can be mass produced by placing the gene for insulin into a bacterium. Large cultures of bacteria make large quantities of insulin which is then harvested. Vaccines can also be made this way. Vaccines are forms of a disease agent that triggers an immune response without producing the disease. For a bacterial disease, modified bacterial cells have traditionally been used as a vaccine, but this sometimes actually causes the illness. The immune response is often triggered by the recognition of a surface protein on the bacterial cell, not the bacteria itself. The surface protein can be made using recombinant DNA, thus creating a safe vaccine.

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