An influenza virus A/PR/8/34(H1N1) challenge boosted the NP-specific immune responses in the surviving mice. To explore the possible mechanisms of immunological protection, 35 days after the challenge with influenza virus A/PR/8/34(H1N1), humoral and cell-mediated immune response changes were explored. The analysis was performed for the surviving mice; no analysis was performed for the mice in the PBS group since none survived. (A) NP-specific IgG antibody titers were tested by ELISA in the surviving mice. (B) Specific T cell responses were tested directly by ex vivo IFN-γ ELISPOT assays against NP147-155 in the surviving mice. (C) In vivo cytotoxicity assays were used to determine the specific lytic potential of effector CD8+ T cells in the surviving mice. The results are representative of two experiments (n = 2-4 per experiment).