Establishment of the barrier between the blood and the brain. The cells in the brain are packed tightly together preventing the passage of most molecules from the blood into the brain. Only lipid soluble molecules or those that are actively transported can pass through the blood-brain barrier. [ GOC:sart http://www.ncbi.nlm.nih.gov/pubmed/20080302 http://www.ncbi.nlm.nih.gov/pubmed/30280653 GOC:aruk GOC:dph ]