It was previously described that the Fab fragment of PN-SIA49 binds to the stem region of HA and neutralizes all tested H1N1 isolates [26, 27] . The heavy chain variable region of PN-SIA49 uses the VH3-23 gene and is paired with a VL1-39 light chain. The nucleotide sequence homology of PN-SIA49 with the germline sequence is 93.11% for the VH gene and 92.30% for the VL gene, demonstrating its origin from a somatic mutation process.