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When a foreign substance (antigen) enters the body, it triggers the immune system and, if successful, it results in the inactivation of the invading organism and, if unsuccessful, creates illness or death. Generally, antigens are proteins. Vaccines are living attenuated (weakened) microorganisms (viral vaccines) or dead microorganisms (bacterial vaccines) or their fractions which mimic the infectious agents without pathogenic consequences. In the development of vaccines the antigens or the components that actually induce immune response are identified and characterized, Some vaccines are very effective, e.g., polio vaccines, whereas others are unsatisfactory, e.g., vaccines of typhoid, cholera, etc. In some cases, fully effective vaccines are not available, e.g., malaria, Massive efforts are in progress to develop a vaccine against AIDS. Through conventional methods, the production of vaccines is costly and occurs in low amounts and, moreover, there is risk of disease development due to occasional presence of active pathogen or reversion of virulence in vaccinated individuals. Moreover, some viruses are difficult to grow in cultured cells, e.g., hepatitis-B virus. Through biotechnology
using animal or human cell cultures, safer vaccine production would increase in near future. Some important types of biotechnological vaccines are:

(i) Purified antigen vaccines: They are non-recombinant vaccines and are prepared by the separation of the antigen from the pathogen, e.g., polysaccharide antigen against pneumonia from Streptococcus pnemoniae, endotoxin (toxoid) for tetanus, etc. Monoclonal antibodies are used to purify antigens with certainity.

(ii) Recombinant vaccines: They are produced through recombinant DNA technology by integrating the gene for the immunogenic proteins in E coli, yeast, cultured animal cells or transgenic plants. Here, DNA segment, necessary for the production of either

a) whole protein (e.g., coat protein gene for hepatitis-B has been cloned in yeast) or

b) part of a protein (e.g., amino acid 114-160 of coat protein for foot and mouth disease virus) necessary for immunogenicity, is cloned. But these protein vaccines (second revolution in vaccines) are very costly and need to be stored at low temperature.

c) A third kind of vaccine is the DNA vaccine (third revolution in vaccines or genetic immunization) in which the isolated gene for immunogenic protein is cloned in a suitable vector; after getting injection, it expresses in the vaccinated individual by the formation of particular immunogenic protein. Influenza vaccine is at advanced stage of testing. They are more safer, more specific, give more potent immune response and are easier to prepare.