For more than a century, the BCG vaccine has quietly protected millions against tuberculosis, but a study from Massachusetts General Hospital now suggests it may carry a second, subtler gift: the capacity to retrain the aging brain's immune cells before Alzheimer's disease takes hold. The mechanism, known as trained immunity, does not simply amplify the immune response but reshapes it, placing dormant brain cells on a broader, more vigilant alert. The finding arrives with a sobering caveat — this retraining appears to matter only when the brain's defenses are still intact enough to act on it,