Mouse Ig2a, IgG2b, and IgG1 are considered equivalent to human IgG1, IgG3 and IgG4, respectively (43), the major subclass of IgG detected in human serum after influenza infection or vaccination is IgG1 and then IgG2 or IgG3 (44–46). This evidence concerns the gene IGHG3 and influenza.