Professor Andy Sewell talks about our immune system with a special emphasis on T-cells – white blood cells that patrol our bodies scanning for, and destroying, foreign microbes, which he describes as both generals and assassins. He describes how new knowledge  – discovered by his pioneering team based in Cardiff University - allows us to beneficially manipulate the immune system and reveal how autoimmune disease can be caused. This information has resulted in exciting advances against cancer and is providing new concepts for treatment of infectious diseases.