Microbial infections remain a major threat to human health. A better understanding of immune effectors is necessary to identify novel targets for therapeutic intervention and could uncover novel approaches to treat autoimmune diseases. One pathway of the innate immune system that is underexplored in the development of novel host-directed therapies is related to ISG15, a ubiquitin-like modification of the immune system. ISG15 has three functions in our body. Similar to ubiquitin, it covalently conjugates to substrates proteins, it controls interferon-α/-β signaling as a free intracellular molecule and it acts as a stimulating cytokine in the interferon-γ pathway. The present project aims to characterize nanobodies directed against ISG15 to modulate its intra- and extracellular function. By combining biophysical methods and epitope mapping with extensive in vitro and in vivo validation experiments, this project aims to validate ISG15 as a potential drug target and to generate proof-of-concept that nanobody-based approaches against ISG15 can be used to treat autoimmune disorders and microbial infections.