The brain’s immune system has long been thought to exist independently from the rest of the body, complete with its own specialized immune cells and a blood-brain barrier that limits what can travel into the brain.

Now, Stanford researchers have found that aging brings with it a large influx of immune cells into the brain, a discovery that not only upends current thinking but could also open new avenues for treating neurological disease. The researchers describe their results in the journal Nature.

“We usually think of the brain as a closed system,” said Julia Belk, a postdoctoral scholar in pathology at Stanford Medicine and first author on the new study. “What we found is that actually a lot of immune cells enter the human brain during aging.”