FIG. 5.

Ratios of serum anti-NP IgG1 and IgG2a Abs in BALB/c IFN-γ^−/− and WT mice. Mice were immunized with influenza virus A/Udorn (H3N2) by TRT. Four weeks later, the mice were challenged with the heterologous mouse-adapted A/PR/8/34 (H1N1) influenza virus. Sera were collected before and 3 days after challenge and assayed for levels of anti-NP IgG1 and IgG2a Abs as determined by ELISA. The values represent the ratios of the mean and SEM of antibody titers for five mice in each group.