After attachment to the receptor on the target cell, influenza virus is internalized by receptor-mediated endocytosis. Conformational changes of hemagglutinin triggered by the low pH in the endosome activate viral and endosomal membrane fusion. In this step, antibodies, which bind to the non-receptor binding region of HA, could interfere with the low-pH induced conformational change in the HA molecule required for the fusion. Inhibition of the fusion between viral and endosomal membrane proteins mediated by such antibodies prevents release of the ribonucleoprotein complex (RNP complex) into the cytoplasm of the target cell, resulting in the inhibition of viral replication ( Figure 2C The interaction of opsonic complement proteins with complement receptor on macrophages (CR) increases the rate of phagocytosis of macrophages, causing direct virolysis or improvement of antibody-mediated inhibition of virus attachment to host cells [26, 27] . However, contribution of complement to the protective capacity of antibodies is contradictory, since it was shown that passive transfer of murine polyclonal anti-eM2 serum into C3-negative mice had protective effect [24], while human monoclonal anti-M2 antibodies could not protect complement-depleted mice [25] . It should be noticed that though some antibodies directed to conserved antigens such as M2 do not prevent infection by direct binding to virus, they can contribute to an earlier recovery from the infection by indirect antibodymediated mechanisms after binding to Fc-receptors on macrophages or NK cells. It is possible that the same mechanism of protection is mediated by antibodies to HA2 glycopolypeptide (HA2 gp), a conserved part of HA. They also do not prevent infection, but their strong protective potential has been proved in vivo [28-31]. For this reason understanding the role of the Fc effector function of antibodies in the clearance of influenza infection is required.