Figure 1. Structures of influenza A group 1 (A) and group 2 (B) hemagglutinin. Published crystal structures for H1N1 HA (pdb 3AL4) and H3N2 HA (pdb 1MQM) were compared by simplifying the trimers into “pretty” monomer formats revealing the tertiary positions of the sequential N-terminal HA1 region (orange), pre-fusion (light blue) and fusion region (yellow; with arginine 329 shown in red) loops, anti-parallel β sheet (royal blue) and finally the α-helical stalk region (green). Shown just below the pretty models are the corresponding 3D crystal structures with the discontinuous epitopes of the lead group 1 (mAb 53) and group 2 (mAb 579) antibodies, determined by PepScan methods. The identified epitope regions are colored according the fusion region domains as described above.