Shimon Sakaguchi described in 1995 a population of CD4+CD25+ T lymphocytes with autoimmunity-suppressing properties, thereafter called regulatory T cells or Tregs. His discovery revealed that the body maintains immune tolerance not only by eliminating self-reactive cells in the thymus but also through active suppression in the periphery. Tregs are today a central therapeutic target in oncology (activating them dampens the immune system; inhibiting them boosts the antitumor response) and in autoimmune diseases. Sakaguchi received the 2025 Nobel Prize in Physiology or Medicine together with Mary E. Brunkow and Fred Ramsdell.