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Hemagglutinin (HA) structure and antigenic binding sites. (A) Structure of HA protein. (B) The five antigenic sites (i.e., Sa, Sb, Ca1, Ca2, and Cb) of H1 (A/California/04/2009; PDB 3UBE). (C) The five antigenic sites (i.e., A–E) of H3 (A/Aichi/2/1968; PDB 2YPG). (D) The four antigenic sites of influenza B viruses (B/Hong Kong/8/1973; PDB 3BT6)
