This approach reveals interesting antigenic features of the HA protein. First, the antibody responses to the 2009 A(H1N1) viral protein HA appear to vary significantly depending on the species, with the goat response being strikingly different from those of mouse and human (Fig. 4) . These results imply possibly different immune responses among animal species, and in the case of human, the specific immunization background may also play a role [13] . Moreover, the antigenic peptides obtained from the screening against the mouse or human antisera are predominantly located in the HA2 region, whereas those from the screening against the goat antisera are predominantly in the HA1 region. Second, five antigenically important regions, R1 to R5 in the order from most membrane proximal to distal (Fig. 5) , are identified. Moreover, except for R5 which is located in the vestigial esterase domain of the HA globular head, all of the other regions are in the HA stem region. Among them, R1 and R2 come together to form a major peak with a FWHM range from residues 387 to 480 (Fig. 4, panels A and E) . Although the peptide P2 corresponding to the R2 region showed no measurable affinity to either mouse or human antisera (Fig. 6) , which suggests that this region might be more conformation-dependent than the rest four regions, it is PLoS ONE | www.plosone.org assigned as a separate region for the following reasons: (1) structurally it is distinguishable from the R1 region (Fig. 5) , and (2) several recent studies have reported the epitopes of monoclonal antibodies targeting H1 subtype HA proteins, which fall into the R2 region of this study [10, 31, 32] (see below).