In this study, we characterize a human mAb, IgG PN-SIA49, directed against influenza viruses and previously described as Fab fragment [26, 27] . IgG PN-SIA49 neutralizes a broad spectrum of viruses belonging to different influenza subtypes (H1, H2 and H5) and is characterized by the lowest mean IC50 values described to date for a heterosubtypic mAb [20, 21, 22, 23, 24, 25] against the influenza subtypes tested in this study (Table S1 ). Furthermore, IgG PN-SIA49 is endowed with a stronger neutralizing activity compared to its monovalent molecule (Table 1) . Indeed, it is well documented in the literature that bivalency of IgG molecule may be an essential feature for the biological activity of a mAb, mostly due to an increase in antibody avidity [33, 34, 35, 36] . The increased avidity may also play an important role in the prophylactic and therapeutic application of an antibody, allowing the administration of smaller amount compared to the Fab fragment. Additionally, the whole IgG molecule has a longer half-life than its fragments, with a consequent prolonged effect of the molecule. For all these reasons, we used IgG PN-SIA49 to treat mice infected with H1N1 and H5N1 viruses and data obtained show that PN-SIA49 protects mice from H1N1 and H5N1 virus lethal challenge suggesting its potential application in treatment of influenza virus infections.