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.

