Characterization of protective antigens and the protective immune response.

The protective immune response to a vaccine may be due to the presence of circulating antibody (humoral immunity), the actions of sensitized T-lymphocytes (cell-mediated immunity), the presence of secretory IgA on mucosal surfaces (mucosal immunity), or a combination of these factors. In general, humoral immunity is felt to be particularly important in protection against extracellular phases of systemic viral and bacterial infections and in protection against toxin-induced diseases. Cell-mediated immunity is particularly important in protection from facultative intracellular bacterial pathogens, intracellular viral infections, fungal diseases, and protozoal diseases. Secretory IgA is important in protecting against those bacterial and viral diseases where the organism must attach to epithelial surfaces in order to produce disease and against diseases induced by toxins produced at mucosal surfaces. To characterize the protective immune response it is necessary to characterize specific protective antigens, to determine the onset and duration of the response, and to demonstrate which aspects of the immune response are responsible for protection. This information is needed to be sure that an in vitro potency assay is relevant to in vivo protection and to be sure that the relevant immune response is being measured when requalifying reference antigens. Determining which component(s) of the immune response is actually responsible for protection may be difficult, particularly for diseases where protection is only partial or where multiple components of the immune response contribute to protection.