The blood-brain barrier is a protective shield that separates the brain from the bloodstream. It consists of tightly packed cells that line the blood vessels in the brain, preventing harmful substances, such as toxins and pathogens, from entering the brain tissue while allowing essential nutrients to pass through. This barrier plays a crucial role in maintaining the brain's stable environment, which is vital for proper functioning. It also poses challenges for medical treatments, as many drugs cannot easily cross the blood-brain barrier, making it difficult to treat conditions like Alzheimer's disease and multiple sclerosis.