All cells, eukaryotic and prokaryotic, face the threat of viral infections and develop anti-viral systems to fight them off. The constant arms race between cells and their viruses leads to the diversification of these defense systems. Thus, hallmarks of immunity in diverse domains such as plants, animals or bacteria are completely different. However, the unveiling of novel anti-phage defense systems in prokaryotes lead us to revisit this paradigm. In this seminar, I will discuss the discovery of novel prokaryotic anti-viral systems that show conservation with eukaryotic ones and discuss the implications of these conservation for both prokaryotes and eukaryotes.