What causes multiple sclerosis?

White blood cells called T cells, which fight disease and infection, instead attack myelin, the fatty substance that surrounds and protects the nerves of the central nervous system. This attack damages the protective myelin sheath and either partially or completely strips the myelin off the nerve fibers (see Multimedia file 2). Because the myelin is supposed to help speed and direct the transmission of impulses along nerve cells, whenever any part of the myelin sheath is damaged, the nerve impulses are distorted. The messages the nerves try to send are delayed and usually misinterpreted by the brain, if the messages get through at all. This interruption of messages is what causes multiple sclerosis (MS) symptoms. Often, the T cells also damage the nerve fibers themselves, causing increased MS symptoms and increased disability over time. (See Myelin and the Central Nervous System.)

It is important for patients to realize that the immune system is not underactive in MS and that it does not predispose them to infections. Rather, the immune system in MS is overreactive to certain environmental agents and responds irregularly by attacking the central nervous system.