CD4 and influenza: In addition to the humoral immune response, CD4+ T cells play an important role in immunity to influenza and are reported to have protective roles that are independent of neutralizing antibody responses.14,15 Various mechanisms underlying cell-mediated immunity to influenza have been reported, including the induction of antigen-specific T and B cells associated with long-term immune memory, characterized by influenza-specific human CD4+ T cell memory responses that are strongly biased toward TH1 cells, producing TNFα, IFNγ, and IL-2 on activation.