In a healthy person, nerve fibers have a protective covering known as myelin that insulates the nerve fibers from damage. In people with Multiple Sclerosis, their immune system erroneously identifies the myelin as a foreign body and sends disease-fighting cells into the central nervous system to destroy myelin. This happens because immune system cells bypass the blood-brain barrier (BBB). BBB is a layer of cells that prevents destructive cells and other substances in the blood from entering the brain and the central nervous system. Damaged myelin can generally be restored, especially during the early stages of the disease, by Oligodendrocytes, which are cells responsible for creating and maintaining myelin. However, gradually oligodendrocytes may be unable to restore the damaged myelin.