Poor paul.fr,
left all alone with such a 'thick' question.
Well, I offer the shortest official definition:
"Blood-brain barrier: A protective network of blood vessels and cells that filters blood flowing to the brain."
The blood-brain barrier is a network of blood vessels with tightly-knit cells that keeps harmful substances from passing from the bloodstream into the brain. This barrier, however, also keeps out many potentially helpful agents, including most chemotherapy drugs and some antibiotics.
When a germ (viruses, bacteria, fungi or parasites) or even cancer cells have been capable to cross it and invade the central nervous system, everything becomes more difficult in terms of treatment; defense cells, antibodies and some drugs have to be carried actively through this 'barrier' to reach their target.
Researchers are experimenting with promising techniques to bypass the blood-brain barrier, but brain cancer remains one of the most aggressive and difficult-to-treat cancers.
Now you probably want to know which vitamin deficiency leads to BBB 'leakage', brain lesions and coma. [:D]
ThikoD