For generations, the brain was believed to stand apart from the body's immune defenses, a sovereign organ behind an impenetrable wall. A discovery from Washington University School of Medicine now reveals that the skull itself harbors organized immune outposts — structures resembling lymph nodes — that have quietly guarded the brain all along. By learning to strengthen these hidden sentinels, researchers improved survival in mice with aggressive brain tumors, suggesting that the architecture of our own bones may hold the key to treating some of humanity's most formidable neurological diseases.