IFNLR1 and infection: Since Ifnlr1−/− mice showed only minimally enhanced susceptibility towards influenza viruses compared with wild-type mice in previous studies (Crotta et al., 2013; Galani et al., 2017; Mordstein et al., 2008), it was assumed that the contribution of IFN-λ to protection against respiratory viruses is minor and becomes detectable only when the type I IFN system is defective (Crotta et al., 2013; Mordstein et al., 2008; Mordstein et al., 2010a; Mordstein et al., 2010b) or when infections are performed with very low doses of virus (Galani et al., 2017).