Unlike mice, human brains recruit fresh immune reinforcements from the bloodstream in middle age, upending decades of neuroscience doctrine about the brain's sealed-off defenses.
For decades, biologists assumed the brain’s immune system replenished itself from birth, sealed off from the rest of the body by the blood-brain barrier — in 2026, Stanford researchers traced shared mutations between blood and brain immune cells, the same trick a genetic ancestry test uses, and found that immune cells quietly migrate from the bloodstream into the human brain starting in middle age, a pattern no one has found in mice or the nonhuman primates tested alongside them
Mind & Matter