Blood-brain  barrier is formed by

Processes of astrocytes (a type of glial cells) wrapped around blood capillaries isolate neurons of the CNS from various potentially harmful substances in blood by secreting chemicals that maintain the unique selective permeability characteristics of the endothelial cells of the capillaries. In effect, the endothelial cells create a blood–brain barrier, which restricts the movement of substances between the blood and interstitial fluid of the CNS.