The blood-brain barrier is a multicellular barrier which regulates the passage of systemic molecules into the brain. It creates a highly regulated environment for the brain, protecting this sensitive organ from blood-derived toxins. The tight junctions from the endothelial cells restrict the paracellular passage of big, charged and hydrophilic molecules, restricting the passage of such molecules through a regulated transcellular route. The pericytes and astrocytes around the endothelial cells give support and regulate the barrier, among others.