Figure 3: Influenza virus-specific IgG subclasses responses to primary (priming) and secondary (challenging) infection in old and young mice. Serum samples were collected from mice prior to and at various time points post-priming (3a) and post-challenging (3b) with 10 MID50 of influenza A/Taiwan/1/86 (H1N1) virus. IgG subclasses (IgG1, IgG2a, IgG2b, IgG3) were determined using ELISA. The levels of the antigen-specific IgG1 and IgG2a were at significantly lower levels in the old mice after day 28 post-priming in comparison with the young mice (P<0.05) (3a). Such a difference was not found for IgG2b and IgG3 (3a). In contrast, the levels of the antigen-specific IgG1, IgG2a and IgG2b for the challenging were significantly lower at all time points in the old mice than those in the young mice (P<0.05-0.001) (3b). Such a difference (P<0.001) was found on day 2 and day 4 but not day 0 and day 6 post-challenge for IgG3 (3b). The data were presented in means ± SE for 4 mice in each group. Compare the young and the old mice at corresponding time points, *P<0.05-0.0001.