Inside the body, MS causes damage that cannot be seen or measured without special medical equipment. The immune system attacks either the fatty parts of the myelin or the protein parts of the myelin. The body may also attack the cells that produce the myelin sheath, called glial cells. When myelin is damaged or missing, large areas of axons affected by the damage are visible as scars or lesions in the central nervous system tissue that build up with repeated repair attempts by the body over time. Lesions appear in different areas of the central nervous system depending on what form of MS a person has.