represents the smaller C-terminal portion of hemagglutinin, which forms a stem-like structure that mediates the anchoring of the globular domain to the cellular or viral membrane. N-terminal part of HA2 gp, termed the fusion peptide, plays a substantial role in the fusion activity of influenza virus. It was demonstrated that the rearrangements of HA as well as the fusion process is temperature-and pH-dependent [49, 50] . At neutral pH, the N-terminus of the fusion peptide is inserted into the inter-space of HA trimer. At low pH, which triggers the fusion process, N-terminus of the fusion peptide is exposed and inserted into the target membrane, allowing the release of the ribonucleoprotein complex into the cytoplasm [51, 16] . Although the epitopes of the [62, 63] . Moreover, it was shown that passive immunization with monoclonal antibodies against HA2 gp, as well as active immunization with recombinant vaccinia virus expressing chimeric molecules of HA, improve the recovery from influenza infection and contribute to a milder course of infection [28, 29] . A recent study showed that increased immunogenicity of HA2 gp could be achieved by unmasking of HA2 gp after removing the highly immunogenic globular head domain of HA1 gp. Headless HA trimers form the conserved HA stalk domain, on which HA2 epitopes are more accessible for B cells than in the native HA. Vaccination of mice with this headless HA immunogen elicited antibodies cross-reactive with multiple subtypes of hemagglutinin and provide protection against lethal influenza virus infection [31] .