Figure 7: Schematic diagram showing the important complementary role of constitutive IFN-β and inducible type I interferon (IFN-α/β) during influenza infection: The respiratory epithelial cell has a basal secretion of constitutive IFN-β to maintain the alveolar macrophage in a primed state for timely production of IFN-α/β in response to influenza infection. The release of constitutive IFN-β by influenza infected respiratory epithelial cell allows the activation of interferon-related genes in the respiratory epithelial cell during the early phase of influenza infection. The inducible IFN-α/β released from infected alveolar macrophages initiates antiviral signaling, triggers apoptosis and limits influenza replication in the respiratory epithelial cells in the early phase of influenza infection. The NS1 protein of H5N1 isolated in humans after 2003 can inhibit release of constitutive IFN-β and production of inducible IFN-β at the pre-transcriptional level and IFN-α/β at the post-transcriptional level.