The HA1 of influenza A virus forms a membranedistal globular domain that contains the receptorbinding site and most antigenic sites recognized by virus-neutralizing antibodies preventing attachment of virus to the host cell. Escape variants with mutation in the antigenic site easily avoid neutralization by existing host antibodies, leading to seasonal influenza outbreaks [45] . In spite of continual antigenic changes of hemagglutinin, common epitopes shared by various strains were identified. Although the degree of sequence diversity between HA subtypes is great, particularly in the HA1 glycopolypeptides, HA2 is its rather conserved part. According to documented results, HA2 has the prerequisite to be one of the potential inductors of protective heterosubtypic immunity [1, 28, 29, [46] [47] [48] . HA2