Influenza viruses bind via their surface HA (hemagglutinin) to sialic acid in alpha 2,3 or alpha 2,6 linkage with galactose on the host cell surface. Sialic acid in 2,6 linkages is characteristic of human cells while 2,3 linkages are characteristic of avian cells. The specificity of influenza HA for sialic acid in alpha 2,6 or alpha 2,3 linkages is a feature restricting the transfer of influenza viruses between avian species and humans. This species barrier can be overcome, however. Notably, passaged viruses adapt to their host through mutation in the receptor binding site of the viral HA gene.