IGKV6-21 and infection: All A(H5N8) and A(H5N2) viruses of North American and Eurasian origin tested preferentially bound to α2,3-SA (avian-like receptor), with minimal binding to α2,6-SA (human-like receptor) (Fig. 2), indicating that the HA of these viruses binds weakly to the dominant receptor of the human upper respiratory tract, α2,6-SA, which is a barrier to human infection.