XK and infection: To validate that hypothesis, mice were first infected via the intranasal route with X31 (H3N2) virus to elicit the antibody (against H3N2) production, and on day 24 post infection, mice were reinfected with NY-ESO-1 S-FLU virus with matched HA/NA (NY-ESO-1 S-FLU [X31]) or mismatched HA/NA (NY-ESO-1 S-FLU [H1 PR8]).