Antigenic sites in IAV HA proteins of H1, H2, and H3 subtypes had previously been characterized by sequence analysis on antigenic variants and amino acid substitutions. These previously identified antigenic sites were mainly located in the globular head in the three-dimensional structure of the HA1 subunit of the HA molecule [36, 37, 38, 39] . For instance, five antigenic sites have been identified in HA of influenza virus A/PR/8/34, a well-known reference strain of H1N1 IAV [39] . Recently, several epitopes were identified in the HA2 unit [34, 40, 41] . Together with these reports, our results indicate that there are more epitopes than what we have imaged and the epitopes of IAV need to be further characterized.