A naturally occurring amino acid called arginine may help the immune system recognize cancer cells and viruses more effectively, according to a new study published in Cell. These proteins act like "ID cards," displaying abnormal or viral proteins on the cell surface so immune T cells can detect and destroy dangerous cells. To understand the effect, scientists examined models of colon cancer, influenza, and SARS-CoV-2. Further laboratory experiments revealed that low arginine caused ribosomes—the cell's protein-making machinery—to stall while producing MHC-I proteins. As a result, fewer immune warning signals appeared on cell surfaces, allowing cancer cells and viruses to evade immune detection more easily.