For most of the 20th century, scientists pictured the immune system as the body’s ever-vigilant army—standing guard against germs, viruses, and rogue cells. Yet one question remained: What keeps this army from mistakenly attacking the body itself? For millions with autoimmune and inflammatory diseases like Type 1 diabetes or lupus, the question is personal—these lifelong illnesses bring painful symptoms and harsh immune-suppressing treatments. This year’s Nobel Prize in Physiology or Medicine honored the discoveries that transformed that understanding. Researchers Mary E. Brunkow, Fred Ramsdell, and Shimon Sakaguchi identified a rare class of immune cells—regulatory T cells, or Tregs. These cells...