The Blood-Brain Barrier (BBB) is a highly selective, semi-permeable barrier that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS). It is primarily composed of endothelial cells, tight junctions, and a basement membrane. The BBB serves as a crucial protective shield, preventing toxins, pathogens, and large molecules from entering the brain, while allowing essential nutrients and gases to pass through.
