Real Dictionary

blood-brain barrier in American English
the barrier created by the walls of the capillaries of the brain that prevents certain substances, as most proteins and drugs, from passing from the blood into the brain tissue and cerebrospinal fluid
noun: a layer of tightly packed cells that make up the walls of brain capillaries and prevent substances in the blood from diffusing freely into the brain: passage across the cell membranes is determined by solubility in the lipid bilayer or recognition by a tr