Brain's Link To Immune System Might Help Explain Alzheimer's
"...The results extend the findings of a landmark study published in 2013. It found that the brain appears to flush out waste products during sleep.
But it wasn't clear how these waste products were draining out of the head. Now it appears that at least some of the waste might be exiting through the lymphatic system...."

http://www.npr.org/sections/health-shots/2017/10/03/555353033/brains-link-to-immune-system-might-help-explain-alzheimers

For centuries, scientists thought the human brain had no direct connection to the body's immune system. Now researchers seem to have found one, and say it may offer clues to multiple sclerosis, too.