When further analyzing HA-specific IgG subclass responses to the primary infection, it was found that both HA-specific IgG1 and IgG2 showed the same pattern as HA-specific IgG, but IgG2b and IgG3 did not show any significant differences between old and young mice, suggesting that the HA-specific IgG difference between old and young mice to the primary infection was mainly resulted from differences of IgG1 and IgG2a responses. The gene discussed is IGHG3; the disease is infection.