Hence, a preexisting CD4+ T-cell pool directed towards commonly conserved epitopes due to previous infections by human IAVs (H1N1 1918–1976, seasonal H1N1 1977–2009, pandemic H1N1 2009–2013, H2N2 1957–1968, and H3N2 1968–2013) - could potentially provide cross-immune protection to the H7N9 virus. This evidence concerns the gene CD4 and infection.