To test if anti-P5 sera cross-react with influenza type B virus, the reactivity of anti-P5 sera with two representative influenza type B virus strains (B/hubeiwujiagang/158/2009,Yamagata lineage and B/heilongjianghulan/116/2010, Victoria lineage) and an influenza type A virus strain (A/H1N1/PR8/34) was examined by Western blot analysis. The results showed that anti-P5 serum reacted well with A/H1N1/PR8/34 virus but not with influenza type B virus strains (Fig. 3C) , further confirming the specificity of the epitope in P5.