IFNG and viral infectious disease: IRGs can be grouped depending on the type of interferon stimulation that induces their expression, including type 1 interferons (i.e., IFNα and IFNβ) which can be produced by almost any cell upon viral infection (32), type 2 interferon (IFNγ), which is secreted by a number of leukocytes to combat infectious agents or cancer (23), and type 3 interferon (i.e., IFNλ), which is secreted by both leukocytes and epithelial cells (33).