Specific antibodies induced by influenza virus infection can neutralize infection by several different mechanisms ( Figure 2 ). They can directly block virus attachment to the target cells by interfering with virus-receptor interaction and thus prevent influenza infection ( Figure 2A ). These antibodies are directed to the globular domain of the surface antigen, hemagglutinin [16] . However, because of high variability of influenza A viruses, neutralization activity of these Abs is limited to viral strains, which are antigenically similar to the inducers of Ab Figure 2 Mechanisms of antibody-mediated neutralization during influenza infection. A. Serum IgG or B. mucosal IgA antibodies specific to hemagglutinin prevent influenza infection by blocking attachment to host cell receptors. C. After binding, the virus is internalized by receptor mediated endocytosis. The low pH in the endosome triggers conformational changes in hemagglutinin that expose fusion peptide located in HA2 required for membrane fusion. In this step, antibodies bound to HA2 block the fusion of viral and endosomal membranes and prevent release of ribonucleoprotein complex into the cytoplasm of target cell. D. Intracellular neutralization of influenza virus through transcytotic pathway of IgA that complex with viral proteins and inhibit assembly of progeny virions. E. Antibodies specific to neuraminidase inhibit release of budding viral particles and further spread of influenza infection by inhibition of neuraminidase activity. production. By contrast, it was shown that mucosal immunity mediated by secreted form of IgA Abs in the upper respiratory tract is more cross-protective against heterologous virus infection than systemic immunity mediated by IgG Abs [17, 18] . The strong cross-protective potential of IgA Abs appears to be the consequence of their polymeric nature, resulting in higher avidity of Abs for the influenza virus compared to the monomeric serum IgG Abs [18] . After synthesis by ASC, dimeric IgA (dIgA) Abs bind to the polymeric immunoglobulin receptor expressed on the basolateral surface of the epithelial cells and are transcytosed to the apical surface, where the poly-Ig receptor is cleaved, secretory IgA are released and prevent infection by blocking attachment to the epithelial cells ( Figure 2B ). Moreover, dIgA Abs are able to bind to the newly synthesized viral proteins within infected cells, thus preventing virion assembly ( Figure 2D ) [19] .