Hemagglutinin (HA) is the major influenza virus target antigen recognized by neutralizing antibodies. It is a surface glycoprotein, synthesized as a single polypeptide, which is trimerized. Each monomer of HA is synthesized as a precursor molecule HA0 post-translationally cleaved by host proteases into two subunits, HA1 and HA2 linked by a single disulfide bond [16] . Cleavage into HA1 and HA2 gp is essential for the infectivity of the virus particle and spread of the infection in the host organism [44] .