Immune responses to influenza virus in rabbits after local immunization. i.--Local and systemic humoral response.

IgA was found to be the predominant immunoglobulin in the bronchoalveolar secretions of rabbits. Significant quantities of IgG antibodies were also present in these secretions after intratracheal immunization with live influenza virus, but IgA/IgG ratios in the secretions were always higher than in corresponding sera, whether the rabbits were immunized or not. The relative excretion coefficients of IgA in these secretions were also found to be higher. In vitro culture of broncho-alveolar wash cells showed active synthesis of all the three major immunoglobulins with IgG as the predominant one. Immunization with live influenza virus induced a higher and more persistant antibody response in the respiratory secretions than the inactivated virus. A rapid and more intense production of IgA and IgG antibodies in these secretions after intratracheal booster antigen strongly suggests the presence of immunological memory in the secretory immune system of the respiratory tract. The role of secretory antiviral antibodies in protection against respiratory infections is also discussed.