The Immune System

The nervous system is not the only system in the body that "talks" to other systems and to itself. Many parts of the body communicate with each other. This is especially true for the immune system, which is responsible for destroying foreign substances such as viruses and bacteria. Most people know about the immune system because they are familiar with the acquired immunodeficiency syndrome (AIDS), in which a virus attacks the immune system and makes it inactive. In MS the picture is different in that the immune system appears to be too active. It sends out "messengers" in the form of specific types of white blood cells that attack myelin as if it were a foreign substance.

The immune system is made up of many different cells that function to protect the body. These cells are made and stored in different parts of the body and make a large number of immunomod-ulating substances. The combinations of cells and substances that may be formed are essentially unlimited, which adds to the complexity of the immune system. Some cells are made in the bone marrow and are called B cells. Some cells are made in other parts of the body such as the thymus gland (over the heart) and in the tonsils (in the throat); these T cells also communicate with and regulate each other. Some cells that suppress reactions are called T suppressor cells; some cells that help reactions along are called T helper cells. Cells in the immune system that target foreign bodies for destruction are called macrophages. Each of these cells has an important individual function; together they create the immune reaction. These reactions usually are beneficial and often life-saving, but sometimes the system malfunctions and produces an autoimmune problem. This is what appears to happen in MS, which therefore often is referred to as an autoimmune disease. other autoimmune diseases include systemic lupus erythematosus (SLE) and rheumatoid arthritis. All autoimmune disease involve the faulty regulation of the immune system, which appears to be overaggressive and may need to be suppressed.

Many things influence the immune system, including exposure to foreign substances, stress, and life itself. A virus may turn the system off, whereas another challenge may turn it on.

Susceptibility to autoimmune diseases appears to be at least partly genetic, so that, although MS itself is not a hereditary disease, a hereditary factor may make an individual susceptible to its development. Approximately 10 to 20% of people with MS have MS in their extended families, a higher rate than would be expected by chance. MS is not a hereditary disease in the sense that most people consider heredity. Clearly, people do not inherit MS, but they may inherit the possibility of developing the disease. The likelihood of developing MS in the absence of its presence in close family members is 1:2000 (0.2 percent). If a parent has MS, the probability that a daughter will develop the disease is 4:100 (4 percent), whereas a son's chances are 2:100 (2 percent). If an identical twin has MS, the likelihood of the other having it is 30%! Again, If MS was solely a hereditary disease, this figure would be 100%, but it does show that genetics plays some role in the development of the disease. Although these numbers are small, they are larger than would be expected if there were no genetic connection. Thus, it appears that one does not inherit MS, but may have a substantial chance of inheriting an immune system that may become overactive if it is stimulated in a specific way. MS is termed a multifactorial disease, which means that more than one factor is involved and that the factors must interact in a highly specific way to result in the disease process.

A distinct possibility exists that viruses may stimulate the immune system and lead to the development of MS in susceptible individuals. Although no virus has been consistently isolated in persons with MS, many investigators believe that a virus originally is responsible for turning on the immune system and making it behave in this abnormal fashion. Because of this, much research is devoted to looking for a viral inducer of MS. Studies of populations of people who appear to be at high risk for MS fuel the idea of a viral origin. For example, the incidence of MS increased dramatically during World War II in the Faroe Islands off the coast of Scotland. other islands off the Scottish coast, the Shetlands and orkneys, had previously had a high prevalence of MS. The difference in prevalence between the two island groups appears to have involved British soldiers who moved to the Faroes during the war. This type of spread of MS follows the pattern of a viral transmission. However, no virus has been found, and the incidence of MS appears to have decreased in both island groups at this time.

The fact that viruses may cause demyelination is demonstrated by the viral origin of the demyelinating disease tropical spastic paraparesis. The search for a viral cause of MS continues. Rubeola, rubella, herpes, and human T-cell lymphotrophic, type I (HTLV-I) viruses all have been considered and eliminated. The herpes 6 virus is being closely looked at. This virus causes a childhood disease that is very common but temporary. Also being closely studied is the common bacterium Chlamydia, which is common in humans, but which usually does not cause symptoms. If history shows a pattern, these organisms are likely to fall by the wayside, as the others have. It is highly likely that if a virus is involved, it has disappeared from the body by the time the immune system has begun its reaction against myelin. The search for a viral cause is further stimulated by the fact that environmental factors appear to be involved in the disease.

People who spend the first 15 years or so of life in areas at a distance from the equator have a much higher risk for developing the disease than do those who spend this time closer to the equator. After the first years, there is no correlation as to where a person lives, but Caucasians appear to be at higher risks than other races.

Research strategies that involve the immune system vary because it is not clear exactly where in the immune process the abnormality occurs. Thus researchers point to many different areas of the immune system in an attempt to change what happens in the MS process.

Even if a cause of MS is not found, it may be possible to halt this disease by intervening somewhere in the immune cascade and halting its progression. This does not mean that a management strategy aimed at allowing a person with MS to do as much as possible given his or her present level of function cannot be developed. This is the principle that underlies symptom management, which has advanced with time, experience, and research. That is what this book is about—making it possible for people with MS to live creative, meaningful, and enjoyable lives.

Multiple sclerosis is unique in that few diseases with the potential to cause disability appear to involve only one system in the body. Except for demyelination, oligo loss, and secondary axonal (nerve) death within the brain and spinal cord, MS leaves the indi-

vidual relatively unscathed. Thus, people with MS usually are quite healthy and have an almost normal life span.

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