Whereas a large number of interacting phenomena contribute to recovery from viral infection, the mechanism of acquired-immunity to reinfection with the same agent appears to be much simpler. The first line of defense is antibody, which, if acquired by active infection with a virus that causes systemic infections, continues to be synthesized for many years, providing solid protection against reinfection. The degree of acquired immunity generally correlates well with the titer of antibody in the serum. Further, transfer of antibody alone, whether by passive immunization or by maternal transfer from mother to fetus, provides excellent protection in the case of many viral infections Thus it is reasonable to conclude that antibody is the most influential factor in immunity acquired by natural infection or by vaccination If the antibody defenses are inadequate, the mechanisms that contribute to recovery are called into play again, the principal differences on this occasion being that the dose of infecting virus is reduced by antibody and that preprimed memory T and B lymphocytes generate a more rapid secondary response.
As a general rule the secretory IgA antibody response is short-lived compared to the serum IgG response. Accordingly, resistance to reinfection with respiratory viruses and some enteric viruses tends to be of limited duration. For example, reinfection with the same serotype of respiratory syncytial virus is not uncommon. Moreover, reinfection at a time of waning immunity favors the selection of neutralization-escape mutants, resulting in the emergence of new strains of viruses such as influenza virus or rhinoviruses by antigenic drift. Because there is little or no cross-protection between antigenically distinct strains of viruses, repeated attacks of respiratory infections occur throughout life.
The immune response to the first infection with a virus can have a dominating influence on subsequent immune responses to antigenically related viruses, in that the second virus often induces a response that is directed mainly against the antigens of the original viral strain. For example, the antibody response to sequential infections with different strains of influenza A virus is largely directed to antigens characterizing the particular strain of virus with which that individual was first infected. This phenomenon, irreverently called "original antigenic sin," is also seen in infections with enteroviruses, reoviruses, paramyxoviruses, and togaviruses. Original antigenic sin has important implications for interpretation of seroepidemiologic data, for understanding immunopathologic phenomena, and particularly for the development of efficacious vaccination strategies.
Was this article helpful?