A study by Stanford University School of Medicine investigators has revealed that immune cells infiltrate the rare newborn nerve-cell nurseries of the aging brain. There's every reason to think those interlopers are up to no good. Experiments in a dish and in living animals indicate they're secreting a substance that chokes off new nerve cell production.
source https://www.lifetechnology.com/blogs/life-technology-news-blog/immune-cells-invade-aging-brains-disrupt-new-nerve-cell-formation