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Blood-brain barrier

(Science: pharmacology, physiology) a protective barrier formed by the blood vessels and glia of the brain. It prevents some substances in the blood from entering brain tissue.

The blood vessels of the brain (and the retina) are much more impermeable to large molecules (like antibodies) than blood vessels elsewhere in the body. This has important implications for the ability of the organism to mount an immune response in these tissues, although the basis for the difference in endothelial permeability is not well understood.