CD4 and influenza: Increased levels of IFNγ-producing CD4 (Th-type 1) and CD8 T cells (CTL) specific for the conserved internal proteins of influenza virus, matrix (M), and nucleoprotein (NP) correlate with protection against influenza in young adults (63, 64) corresponding to the key role of CTL in clearing influenza-infected cells from the lungs.