The HA glycoprotein mediates attachment and entry of the virus by binding to sialic acid receptors on the cell surface. The binding affinity of the HA to the host sialic acid allows for the host specificity of influenza A [6, 7] . Avian influenza subtypes prefer to bind to sialic acid linked to galactose by α-2,3 linkages, which are found in avian intestinal and respiratory epithelium ( Table 2 ) [8] . Human virus subtypes bind to α-2,6 linkages found in human respiratory epithelium [8, 9] . Swine contain both α-2,3 and α-2,6 linkages in their respiratory epithelium, allowing for easy co-infection with both human and avian subtypes (thus acting as a 'mixing vessel' for new strains) [10] . Humans have been found to contain both α-2,3 and α-2,6 linkages in their lower respiratory tract and conjunctivae, which allows for human infections by avian subtypes [9, 11, 12] . The HA glycoprotein is the main target for immunity by neutralizing antibodies.