The secretory γA globulins have been shown to be the active antiviral neutralizing principle in the respiratory tract. These antibodies are part of the system found in other fluids in contiguity with the exterior, the external secretory system. The presence of these γA associated antibodies has been shown to be correlated with protection particularly in the case of certain infections which are localized in their pathogenesis. The production of these antibodies are favored by either natural infection, attenuated vaccine, or by local application of viral antigens directly to the respiratory tract. When this system is inappropriately stimulated, hypersensitivity phenomena may occur. Finally, these observations suggest that the secretory γA globulins serve as a basis for an understanding of resistance mechanisms in the normal, and for the development of new approaches to the use of vaccines.