The isolation of PN-SIA28 has been previously described [19, 20] . In brief, PN-SIA28 was isolated from a 55 year old patient with a negative clinical history of infection from influenza viruses during the past ten years, and with a detectable serum neutralizing titre (half maximal inhibitory dilution 2 ID 50 . = 1:20) against two reference strains belonging to two distinct HA-based phylogenetic groups: the H1N1 strain A/Puerto Rico/8/1934 for group 1 and the H3N2 strain A/Port Chalmers/1/1973 for group 2. It was then expressed in E. coli as Fab fragment to define its features.