Host defenses against influenza virus: the role of anti-hemagglutinin antibody.

In CBA mice the protection provided by transferred immune spleen cells or by antibody has been investigated in immunologically intact, cyclophosphamide-treated and thymus-deprived animals infected with A/PR8 virus. The degree of protection was more closely related to serum antibody levels than to the presence of immune lymphocytes in recipients. Comparison of the protective efficiency of various anti-influenza antisera with different specificities within an influenza A subtype indicated that antibodies recognizing the strain-specific determinants of the influenza hemagglutinin have an important role in protection. Physiologic amounts of transferred antibodies were shown to protect immunodepressed mice, suggesting that, provided a sufficient amount of specific antibodies is secreted, the participation of effectors of cell-mediated immunity is not essential. However, our results suggest that thymus-derived lymphocytes have an indirect role in protection by enhancing, through their helper effect, the secretion of anti-influenza antibodies.