CD40LG and influenza: IgG3, IgA1, and IgM antibodies directed to both HA and NA antigens were linked to non-protective immunity, suggesting that individuals who ultimately developed influenza symptoms mounted an immune response marked by the generation of de novo (IgM/IgG3) and mucosal (IgA1) humoral responses, which targeted several viral strains including H1N1, H3N2, and B viruses.