Oligonucleotides bearing free, uncapped 5′ phosphate group(s) are recognized by RIG-I, leading to the induction of type I IFN, IL-18 and IL-1β production. Bacterial RNA also induces type I IFN production. 5′ phosphate oligonucleotides and bacterial RNA can be used for inducing an anti-viral response or an anti-bacterial response, in particular, type I IFN and/or IL-18 and/or IL-1β production, in vitro and in vivo and for treating various disorders and diseases such as viral infections, bacterial infections, parasitic infections, tumors, allergies, autoimmune diseases, immunodeficiencies and immunosuppression. Single-stranded 5′ triphosphate RNA can be used for inducing an anti-viral response, an anti-bacterial response, or an anti-tumor response, in particular, type I IFN and/or IL-18 and/or IL-1β production, in a target cell-specific manner.