Factors promoting pathogenicity of influenza virus.

Each year, influenza infections cause significant morbidity and mortality in spite of intense research and control efforts. Virus is transmitted primarily by aerosolization of respiratory secretions, and infection can be caused by a small number of viral particles. Following inhalation, virus is deposited on the mucous blanket overlying the target epithelial cell. Mucociliary clearance is the normal mechanism by which deposited particulates are removed; immunoglobulins at the airway surface also play an important role in neutralization of virus. Influenza virus is very efficient at overcoming host defenses, primarily due to the function of two surface glycoproteins, hemagglutinin and neuraminidase. Both glycoproteins have antigenic sites that undergo variation. Hemagglutinin also contains cell binding sites, while neuraminidase prevents interference of mucins with cell binding. Although other factors may contribute to infectivity of the influenza virus, antigenic variation of hemagglutinin is the single most important factor. Influenza vaccination is the most effective means presently available to decrease the attack rate and impact of this infection on society.