Definition: The blood-brain barrier is the semipermeable layer of cells within the capillaries of the central nervous system. When functioning normally, the blood-brain barrier prevents foreign invaders (such as viruses) and immune cells from passing out of the bloodstream and into the central nervous system. It also allows oxygen and essential nutrients to enter the central nervous system.
In multiple sclerosis, immune cells are allowed to enter the central nervous system, implying that the blood-brain barrier is damaged or compromised in some way. These immune cells, called T lyphocytes, attack the myelin in the brain and spinal cord, causing the lesions which lead to MS symptoms.
Tysabri (natalizumab) is a disease-modifying drug which works by preventing immune cells from crossing the blood-brain barrier.
