Figure 2: Reduced antibody response to primary (priming) and secondary (challenging) influenza virus infection in old mice is selective for IgG. Serum samples were collected from mice prior to and at various time points post-priming and post-challenging with 10 MID50 of influenza A/Taiwan/1/86 (H1N1) virus, respectively. HAspecific IgG, IgM, IgA were measured using ELISA. IgG antibody response to the influenza virus infection were lower in old mice than in young mice. The difference became significant (P<0.05) on the 28th day after the primary infection (2a) and all time points for the secondary infection (P<0.001) (2d). In contrast, the antigen specific IgM and IgA responses were all higher in old mice than in young mice (P<0.05 and 0.01) (2b, 2c, 2e and 2f) beginning on the 7th day following priming with virus. Compare the young and the old mice at corresponding time points, *P<0.05-0.001.