The blood-brain barrier (BBB) is a protective layer that lines the inner surfaces of the blood vessels inside the brain. It is a term used to describe the unique properties of the microvasculature of the central nervous system (CNS) . The BBB is a selective semi-permeable membrane between the blood and the interstitium of the brain, allowing cerebral blood vessels to regulate molecule and ion movement between the blood and the brain. The BBB is composed of endothelial cells (ECs), pericytes (PCs), capillary basement membrane, and astrocyte end-feet, all of which aim to shield the brain from toxic substances, filter harmful compounds from the brain to the bloodstream, and supply brain tissue with nutrients. The endothelial cells lining the inside of the brains blood vessels are tightly packed together, forming the blood-brain barrier. The BBB allows only certain substances like water, oxygen, and small lipid-soluble substances to easily cross from the blood into the brain, preventing toxins, pathogens, and other potentially dangerous substances from crossing from the circulation. The BBB is essential for proper neuronal function and also protects the neural tissue from toxins and pathogens, and alterations of these barrier properties are an important component of pathology and progression of different neurological diseases.