Blood-brain barrier (BBB)

The blood-brain barrier (BBB) is a highly selective, semipermeable border that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS). It acts as both a physical and functional barrier, tightly regulating which substances can pass from the bloodstream into brain tissue. The BBB is primarily formed by endothelial cells lining cerebral capillaries, connected by tight junctions, surrounded by pericytes and astrocyte end-feet. It allows passive diffusion of small hydrophobic molecules and active transport of essential nutrients, while restricting entry of pathogens, toxins, and large solutes. Disruption of BBB integrity can occur during infections, trauma, stroke, or inflammation, leading to increased permeability and potentially severe consequences. The BBB is essential for normal neural function but also presents challenges in drug delivery to treat CNS diseases due to its restrictive nature.