The blood brain barrier is like a wall that has holes in it. It allows good substances to cross from the blood vessels into the brain. At the same time, it keeps out harmful substances like bacteria and some toxins. According to this article, researchers think that some illnesses may result from the breakdown of the blood brain barrier.
The article discusses a research study showing that untreated obstructive sleep apnea (OSA) can breakdown the blood brain barrier. The researchers think that the low oxygen levels associated with OSA are responsible for the breakdown. But the breakdown may be what causes some of the symptoms of OSA, such as memory or mood problems. The researchers also think that repairing the blood brain barrier may improve these symptoms in patients with OSA. I also wonder if treating the OSA repairs the blood brain barrier - more studies will be needed to figure that out.