Retrovirus

The specificity of vaccines to a particular antigen is critical in order to mount an appropriate immune response. In order to function effectively, both plasma cells and activated T cells must recognize a particular pathogen. Since HIV frequently mutates, several forms of the virus may be present and therefore a single drug may target one form but not another. The flu vaccine is developed early in the flu season after the virus affecting the first victims is characterized. If a person is infected with a different strain or with a mutated vaccine-related strain, the vaccine is ineffective.