the risk of developing MS, but the precipitating event that somehow results in the immune system's attack on the nervous system remains unknown. The attacks may be few and far between with little or no impact on a person's ability to function, or they may cause a rapid progression toward severe disability. Most people with MS fall between these extremes and, on average, live only a few years less than the general population.
MS is probably an autoimmune disease, meaning that the body's natural defenses are turned against itself. Instead of destroying foreign cells, the immune system destroys the body's native cells. For example, in the autoimmune disease, Type 1 diabetes, the insulin-producing cells of the pancreas are destroyed. In MS, the myelin sheath that insulates nerve cells is destroyed (see Figure 1.2). Without the myelin sheath, nerve cells lose their ability to conduct nerve impulses. As the number of damaged nerve cells increases, the body loses its ability to perform the functions controlled by these cells.
This attack on the myelin sheath is believed to be orchestrated by blood-borne immune cells that invade the brain through the blood-brain barrier, the physical-chemical barrier that surrounds the brain and normally protects it from foreign and toxic substances circulating in the blood. The brain is thus normally resistant to infections that afflict the rest of the body. MS is one of the few diseases in which the blood-brain barrier is breached.