Schematic illustration of blood-brain barrier (BBB) structure. (A) Brain endothelial cells (ECs) are lined to form blood vessels. Pericytes wrap around the vessel and astrocytes extend endfeet to contact with ECs. Neurons and microglia cooperate with each other to form more functional neurovascular unit (NVU). (B) The barrier accounts for limited paracellular transport due to tight junction proteins (e.g., zonula occludens 1 (ZO-1) and claudin) and transcellular transport due to efflux pumps (e.g., P-glycoprotein).