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Your brain's immune cells were supposed to be sealed in for life. In humans, blood cells start joining them at middle age.

A Stanford team traced shared mutations between blood and postmortem brain tissue — the same logic an ancestry test uses — and found circulating immune cells taking up residence in the aging human brain as functional microglia. Mice don't do it. The non-human primates tested didn't either. This is not a story about a leaky barrier.

BriefLookout · 26 Aug 2026 · 4 min read