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

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

For most of the modern history of neuroscience, the working assumption was that the brain’s resident immune cells, called microglia, were installed before birth and stayed put for life. The blood-brain barrier, the tightly regulated layer of cells lining the brain’s blood vessels, was understood to keep the brain’s immune defences separate from the ones patrolling the rest of the body, replenished from within rather than topped up from outside. …