Previously most influenza HA antibodies have been found to recognize epitopes in the globular head and interfere with binding to target cells [6] [7] [8] [33] [34] [35] [36] , for example, the classical five antigenic sites (Sa, Sb, Ca 1 , Ca 2 , and Cb) [6, 7] . However, technically the monoclonal antibodies used in these previous studies were obtained from murine hybridoma cells by radioimmunoassay (RIA) or hemagglutination inhibition (HI) assay. The nature of these assays favored the isolation of antibodies that bound predominantly to the globular head of the virus and inhibited binding of the virus to the host cells. In this current study, however, complex antisera were used, which should contain antibodies recognizing other regions of the HA protein. On the other hand, only one antigenic region (R5) is identified in the globular head in this study, possibly because the epitopes in this head region are more conformation-dependent and may require longer peptides or even a complete domain of HA1 to be displayed for full antigenicity (at least for the mouse serum and human plasma samples). It is then noteworthy that the head region, especially the area around the receptor binding site, contains mostly beta structures, which more likely require stabilization by long-range interactions than the helical structures in other regions of HA (Fig. 5) . However, the peptide library constructed in this study was enriched at 100-500 bp (or less than 170 aa), which was likely insufficient to map the conformational epitopes. Nonetheless, the current screening method is a valuable complementation to the more classical approaches that are biased toward full HA1 or HA2 domain.