Genetic engineering has been used to make new, highly effective types of vaccines to protect animals or humans from disease. The approach is to use molecular techniques to "disarm" the pathogen, or to express single proteins of the pathogen in a harmless bacterium or the vaccinated animal itself. These strategies elicit a strong immune response against the pathogen without exposure to the disease-causing organism. With the advent of molecular techniques, genetically modified organisms have been produced that stimulate strong immunity with few of the risks of killed or attenuated vaccines. At present, several different classes of genetically modified vaccines are either in testing or already on the market.

The first approach stimulates an immune response using only a single protein or protein fragment from a pathogen synthesized in genetically engineered cells.

A second type approach is to use genetic engineering to create a "live attenuated vaccine" by specific deletion of genetic information which renders the disease-causing organism harmless by taking away the genes coding for those proteins necessary to cause disease, while preserving the genes for proteins that stimulate the immune system.

A third type of genetically-engineered vaccine can be produced by inserting immune-inducing genes from a pathogenic organism into a vector that is not capable of causing disease in the vaccinated animal. This approach has great potential as a vaccine because it stimulates cell-mediated as well as humoral (antibody-producing) immunity.

A fourth type of vaccination approach is to inject a defined segment of DNA into a patient or an animal. The DNA is chosen to code for one specific antigenic protein from a pathogen. The protein made is chosen to elicit an immune response but not to cause disease. Initial concerns that the foreign DNA may become incorporated into the host's chromosomes have not been demonstrated, although the possibility has slowed commercial application.

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