The blood-brain barrier (BBB) is a barrier that protects cells and other components of brain tissue from the brain’s blood vessels (capillaries). Structurally, it’s made up of endothelial tight junctions that only permit small molecules, fat-soluble molecules, or gases to pass. This functions to protect the brain from pathogens and toxins present in the blood that could cause brain infections. At the same time, the BBB also allows vital molecules (hormones, nutrients, water)  to reach the brain. One issue with the tight junctions is that it prevents a lot of potential drugs from being able to reach the brain.