The blood-brain barrier (BBB) is a specialized structure that surrounds the blood vessels in the brain and acts as a filter to protect the brain from harmful substances. It is composed of a layer of tightly packed cells that line the walls of the blood vessels in the brain.

The BBB serves several important functions. Firstly, it regulates the passage of nutrients, oxygen, and other essential substances into the brain, while preventing the entry of harmful substances such as toxins and pathogens. Secondly, it helps to maintain a stable environment within the brain, by regulating the levels of ions, neurotransmitters, and other substances that are necessary for normal brain function.

The BBB is essential for maintaining the health and function of the brain, but it can also pose challenges for the development of drugs and other therapeutic agents that need to cross into the brain to be effective. Because the BBB limits the entry of substances into the brain, drug developers must carefully design drugs that can cross the barrier or develop delivery methods that can bypass the BBB altogether.