The blood-brain barrier (BBB) is the protective layer that isolates the central nervous system from the bloodstream but allows the exchange of substances necessary for it to function. The blood-brain barrier consists of the cells of the inner lining of blood vessels and other cells behind them pressing together very tightly.

Blood-brain barrier links:
Multiple Sclerosis, The Blood Brain Barrier, and New Treatment 

MS Glossary
All About Multiple Sclerosis