The most important protective antigen on the surface of influenza virus is HA, whose structure can be divided in two distinct regions: the globular head, responsible for the binding to the sialic acid, and the stem region that contains the fusion peptide and the membrane anchor domain. On the globular head, constituted by the HA1 subunit, lie several epitopes targeted by neutralizing antibodies [18] . However, mAbs recognizing this region are of restricted application due to the antigenic drift that this region encounters [18] . In contrast, the stem region of HA, formed mostly by the HA2 subunit, is relatively conserved among different influenza A subtypes [19] and indeed could represent an universal target for the development of cross-neutralizing monoclonal antibodies.