The blood-brain barrier.

In animals, the central nervous system (CNS) plays a crucial role as the primary organ responsible for integrating environmental information and coordinating the responses of the organism. To protect its intricate and delicate structure, the CNS is enclosed within the rigid confines of the skull, an effective defence against physical trauma. However, threats to the brain also arise internally, in the form of toxins, pathogens, or activated immune cells. This has driven the evolution of specialised vascular adaptations that tightly control exchanges between the blood and the CNS. These 'blood barriers' are tailored to the specific structure they protect, whether it's the brain, spinal cord, retina, or cerebrospinal fluid (CSF). In this primer, we focus on the structural and functional features of the blood-brain barrier (BBB) (Figure 1A).