Immunologic and virologic aspects of secretory immune system in human respiratory tract

Many external mucosal surfaces in man are replete with immunoglobulin containing plasma cells and thymic dependent (T) lymphocytes. Immunization with viral vaccines administered via different routes have indicated that viral specific secretory immunoglobulins particularly of gamma A class are synthesized locally in the external mucosal surfaces. Local availability of viral antigens especially after local mucosal immunization appears to be the most effective means of inducing viral specific secretory antibody responses in the respiratory tract. Similarly, local induction of specific cellular immune responses in the respiratory tract and tonsilar lymphoid cells has been demonstrated after intranasal immunization with viral vaccines. Generally, the locally induced secretory antibody and cell-mediated immune responses in the respiratory tract appear to be independent of the immune response in the systemic sites, with little or no contribution from circulating immunoglobulins and T-cells. Information obtained after natural or vaccine induced infections with polio, influenza, measles, rubella and other viruses suggest that the outcome of a reinfection challenge in the respiratory and alimentary tracts is determined to a major extent by the presence and level of pre-existing secretory antibody. Although the precise role of locally induced cellular immunity in protection against viral infection remains to be determined, these observations suggest that the mechanism of immunologic defence in external surfaces may be mediated largely through specific secretory immunoglobulin and cellular immune response.