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→Vaccine Antigens
==Vaccine Antigens==
==Vaccine Antigens==
Potential antigenic substances include:
===Whole Organism===
'''Live Attenuated (LA) vaccines''' include the organism but in an altered form - virulent organisms cannot be used as vaccines as they have the potential to cause disease. Virulence is reduced by growing the organism in altered conditions (e.g. in cells or eggs), so that it is less able to replicate when introduced into the host, and is therefore less likely to cause disease. Virulence can also be reduced by genetic engineering, or by using naturally occurring avirulent strains.
LA vaccines produce a superior response to disease than using killed organisms as the dose of antigen is larger and more sustained, and the response takes place at the site of natural infection, producing a greater local response than with killed organism vaccines. Examples include:
*The current vaccine for Tuberculosis (called BCG) contains an attenuated form of a mycobacteria
*Vaccines for Leishmaniasis
*Vaccines for parainfluenza virus 3 of calves is developed to be temperature-sensitive so that it grows at 34 C in the upper respiratory tract but not at 38 C in the lungs
**E.g. The current vaccine for Tuberculosis (called BCG) contains an attenuated form of a mycobacteria
**E.g. Vaccines for Leishmaniasis
**E.g. Vaccines for parainfluenza virus 3 of calves is developed to be temperature-sensitive so that it grows at 34 C in the upper respiratory tract but not at 38 C in the lungs
'''Killed inactivated organism or toxin (toxoid)''' are useful where virulent and toxic organisms cannot be used as vaccines as they would cause disease. Organisms can be killed using radiation or chemicals so that they still possess the antigens to stimulate an immune response, but the organisms are unable to replicate inside the host. Alternatively, toxins are inactivated to produce a toxoid which will still have the antigens needed to produce an immune response but will not be harmful to the host. Two doses (for an explanation on the [[Lymphocytes#T cells|T cell]] response click [[B cell differentiation#T-Cell Dependent and Independent Responses|here]])
antigens to stimulate an immune response, but the organisms are unable to replicate inside the host
**1:4000 formaldehyde is the current preparation
**1:4000 formaldehyde is the current preparation
**Inactivants containing azuridines and beta propiolactone are being developed which do not leave a persistent infectious viral fraction (like formaldehyde)
**Inactivants containing azuridines and beta propiolactone are being developed which do not leave a persistent infectious viral fraction (like formaldehyde)