IGKV6-21 and infection: Influenza virus HA is the surface protein involved in binding host cell sialic acid in order to initiate infection, and the specificity of the HA binding depends on the virus and host species, with human and swine viruses preferentially binding α2,6-linked sialyl receptors whereas avian viruses bind α2,3-linked sialyl receptors [reviewed in ref 27].