Influenza species tropism
It is generally accepted that human influenza viruses preferentially bind to host glycoproteins/glycolipids containing sialic acids in alpha 2,6 linkage, whereas avian influenza viruses bind to those containing sialic acids in alpha 2,3 linkage.
Recent studies have shown that the Human traches and alvoeli epithelial cells are mainly expressing alpha 2,6 sialic acids, when only alveoli are expressing alpha 2,3 sialic acids . This could explain why avian viruses could succesfully infect humans only when breathed at high doses. Indeed the trachea and bronchioles are much easier to reach than the alveoli for aerial-transmitted virus.
These results are debated, and investigations are needed to understand the whole picture