NEUTRALIZATION TEST FOR VIRUS
Neutralization of a virus is defined as the loss of infectivity through reaction of the virus with specific antibody. Virus and serum are mixed under appropriate condition and then inoculated into cell culture, eggs or animals. The presence of unneutralized virus may be detected by reactions such as CPE, haemadsorption/haemagglutination, plaque formation, disease in animals. The loss of infectivity is bought about by interference by the bound Ab with any one of the steps leading to the release of the viral genome into the host cells. There are two types of neutralization;- Reversible neutralization - The neutralization process can be reversed by diluting the Ab-Ag mixture within a short time of the formation of the Ag-Ab complexes (30 mins). It is thought that reversible neutralization is due to the interference with attachment of virions to the cellular receptors eg. the attachment of the HA protein of influenza viruses to sialic acid. The process requires the saturation of the surface of the virus with Abs. Stable neutralization - with time, Ag-Ab complexes usually become more stable (several hours) and the process cannot be reversed by dilution. Neither the virions nor the Abs are permanently changed in stable neutralization, for the unchanged components can be recovered. The neutralized virus can be reactivated by proteolytic cleavage. Stable neutralization has a different mechanism to that of reversible neutralization. It had been shown that neutralized virus can attach and that already attached virions can be neutralized. The number of Ab molecules required for stable neutralization is considerably smaller than that of reversible neutralization, Kinetic evidence shows that even a single Ab molecule can neutralize a virion. Such neutralization is generally produced by Ab molecules that establish contact with 2 antigenic sites on different monomers of a virion, greatly increasing the stability of the complexes. An example of...
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