The 2025 Nobel Prize in Physiology or Medicine honored foundational work revealing regulatory T cells (T‑regs) as the immune system’s brake on autoimmunity. Stockholm’s Royal Swedish Academy and multiple institutional announcements named Shimon Sakaguchi, Mary Brunkow and Fred Ramsdell for discoveries that define peripheral immune tolerance. The award re-centers academic and industry focus on leveraging T‑regs for autoimmune disease and cancer immunomodulation. The laureates’ studies traced how T‑regs suppress self-reactive lymphocytes and maintain immune homeostasis, providing mechanistic targets for drug developers pursuing tolerance-promoting therapies. Regulatory T cell biology underpins both efforts to dampen autoimmunity and to transiently modulate tolerance in cancer. For developers, T‑reg biology is a translational bridge: understanding suppression pathways enables both inhibitory antibodies and cell‑based modulation strategies. A one-sentence clarification: regulatory T cells (T‑regs) are a specialized subset of CD4+ T cells that suppress immune responses to prevent tissue damage and autoimmunity.