As a requirement for infection, the homotrimeric HAs play a key role in binding to the host sialic acid (SA) receptors and membrane fusion. The nascent HA of all subtypes consists of conserved structures, including the signal peptide, the cytoplasm domain, the transmembrane domain and the extracellular domain [16] . The mature HA monomer can be cleaved by proteases into the global HA1 and stalk HA2 subunits [17] . When IVs are located in the host digestive tract or respiratory tract cell, the receptor binding domains (RBDs) at the tip of HA1 bind to the SAa2-3Gal or SAa2-6Gal receptors, which are essential for endocytosis [18, 19] . HA unfold and expose the interior HA2 subunits in the acid environment, then the fusion peptides in HA2 insert themselves into the host membrane (viral membrane fusion) [20, 21] .