With some exceptions, autoimmune diseases tend to affect one sex more than the other; specifically, far more women than men. For some time, physicians and researchers have been asking why this is so.
The most popular theory to date has been that female hormones set the stage for diseases such as systemic lupus erythematosus, Sjogren's syndrome, and rheumatoid arthritis. But more recent research calls this theory into question-and suggests that there may be other reasons for the greater incidence of autoimmune disease in women, an issue sometimes referred to as sex predominance. Possible explanations may be roughly divided up into four areas: (1) hormone theory, (2) environmental factors, (3) genetic influences, and (4) whole organism factors.
Discussion of these potential causes is sometimes complicated by disagreement about the definition of an autoimmune disease. For the most part, however, there is general agreement that the following are autoimmune diseases: system lupus erythematosus, rheumatoid arthritis, Sjogren's syndrome, scleroderma, primary biliary cirrhosis, chronic active hepatitis, Graves disease, Goodpasture's disease, hemolytic anemia, idiopathic thrombocytic purpura and Hashimoto's thyroiditis.
Other diseases that many physicians would put in this category include: ankylosing spondylitis, Lyme disease, pemphigus, vitiligo, myasthenia gravis, multiple sclerosis, and juvenile onset diabetes. A number of other illnesses may have features that are similar to those of autoimmune diseases, but they are not labeled as autoimmune diseases.
1. Hormone Theory
Hormone theory refers to the traditional belief that estrogen production puts women at a greater risk for developing an autoimmune disease. In tests conducted in the laboratory, estrogen has been demonstrated to make women's immune systems "over-react" when compared to men's.
This finding supports the standard definition for lupus and other auto-immune diseases, in which the immune system is said to over-react to the body's own tissues. It also appears to explain the ratio of incidence in a disease like lupus, in which nine women get the disease for every one man.
However, there are autoimmune diseases that occur more frequently in men than in women, including Goodpasture's syndrome, in which the body creates antibodies that attack the lungs and kidneys. This disease occurs in one woman for every three men.
Another problem with the hormone theory is disease severity. If women are more likely to develop lupus because they produce estrogen, they could be expected to become more seriously ill. But, overall, men and women with lupus experience the disease with the same range of severity.
If estrogen were the deciding factor in who gets autoimmune diseases, it would also be reasonable to expect that similar diseases would have similar male-female ratios. However, there are a number of diseases that share characteristics with lupus (9:1) that have very different ratios, including Goodpasture's disease (1:3), idiopathic thrombocytopenic purpura (2:1), and hemolytic anemia (2:1).
If hormones and immune response are linked, then one could also expect women to react differently than men to infections and immunizations. However, there appears to be no significant difference. Moreover, hormonal changes that occur when menstruation begins, during pregnancy, and with menopause, do not seem to affect the course of autoimmune disease. Neither does the use of birth control pills or hormone replacement therapy.
2. Environmental Factors
As these arguments against the role of hormones in explaining sex predominance and autoimmune disease have become more widely discussed, researchers are paying more attention to environmental factors. Some diseases that have been classified as autoimmune diseases are clearly caused by exposure to external toxins -- medications or environmental pollutants. The differing roles of men and women at home and in the workplace help to explain who is exposed to these toxins.
Scleroderma, for example, is seen disproportionately in male gold and coal miners who are exposed to high levels of silica. Drug-induced lupus is a long-recognized disease that occurred in the 1960s and 1970s, mostly in men who were given certain drugs to treat heart disease, and still occurs today, but less often since better drugs are available.
Other examples of autoimmune disease caused by exposure include an epidemic of a disease that resembled scleroderma in Spain in the 1970s. In this case, 10,000 people, mostly women who were cooking at home with contaminated oil, developed the illness. While both men and women ate the cooked foods, women tasted the food as it was cooking, at a point when the heat had not yet destroyed the contaminant. By the time the food reached the table, the contaminant was destroyed, and no longer harmful.
In another instance, men who cleaned manufacturing vats used to make polyvinyl chloride (the plastic that is used in so many products that we use on a daily basis) developed scleroderma. It was later found that they were exposed to a monomer in the air, a toxic chemical compound that joins with other compounds during heating to become polyvinyl chloride.
The role of environmental factors may also be seen in children. In those under 12, for example, more boys develop Lyme disease than do girls. The reason for this difference can be found in the way children play. More boys play outside more of the time, and in doing so, increase their risk of exposure to disease-carrying ticks in wooded areas. (Looking at this example, one could come to the conclusion that a preference for a certain type of activity might be determined by hormones, which in turn could influence exposure to the environmental factor.)
In an area of Brazil, a disease that closely resembles pemphigus foliaceus, an autoimmune skin disorder characterized by blisters, has been associated with a certain kind of black fly that lives near the river. Men who fished in this river were more likely to develop the disease than women who had less exposure to the flies.
These examples support the idea that exposure to a toxin determines who will develop the disease, rather than the sex of the patient. Unfortunately, it does not yet help to explain female predominance in a disease like systemic lupus erythematosus. Researchers have looked at exposure to a number of products that women use with greater frequency, such as hair dye and lipstick, but have not identified any cause-and-effect link.
The role of behavior can also be considered to influence the development of disease. Examples include osteoporosis, which can be modified by doing weight-bearing exercise. Coronary disease, including strokes and atherosclerosis, can be affected by changes in diet and reduction of cholesterol levels. And, injury-related osteoarthritis risk can be reduced by avoiding extreme sports. The risk of developing Reiter's disease-a kind of reactive arthritis sometimes caused by venereal disease-can also be reduced by modifying behavior.
It seems clear, therefore, that behavior can cause illness, but we haven't identified the specific behaviors involved with most autoimmune diseases.
3. Genetic Influences
Genetic differences between men and women have also been considered as an explanation for why one sex gets a disease more frequently than the other. Researchers have learned some interesting facts about these differences. Among them:
As fascinating as this information is, scientists have not yet figured out its significance. But we can expect much more study of these phenomena.
4. Whole organism Issues
Whole organism issues may also be a part of the picture-those that have nothing to do with sex or genes. For example, osteoporosis is, in large part dictated by body size. Very heavy people don't get it, very thin people do. Moreover, tall, heavy people are less likely to have osteoporosis than are small, heavy people, since the disease relates to the size of the bones and how much calcium is lost from them.
The likelihood of a woman developing breast cancer may be influenced by how many children she has, and Alzheimer's disease appears to be linked with aging, although it is not understood whether it is part of the process itself, or something that takes 60 or 70 years to develop. Study of the aging process and autoimmune disease is underway as well. Can signs of the disease be detected in the bloodstream well before the patient has any symptoms? And, if so, are there ways to slow the progress of the disease or stop it?
Looking at all of these factors, Dr. Lockshin concludes that a variety of factors may explain why more women get autoimmune diseases than do men. Why is there a nine to one ratio in a disease like lupus? Why does rheumatoid arthritis occur in two women for every one man? Why do more men than women develop Goodpasture's syndrome? It could be environment, hormones, behavior, genetic differences, or a combination of some or all of these factors. The information available to us now is part of a continuing discussion that promises to yield new ways of thinking about and approaching autoimmune disease over the coming decade and beyond.
If you are interested in reading more about the relationship between sex and disease, you may be interested in the following:
Exploring the Biological Contributions to Human Health: Does Sex Matter? This book compiles a vast range of writings on the role of sex in a range of diseases, not just autoimmune illnesses. It is available for purchase or for free on-line at www.nap.edu/catalog/10028.html
The X in Sex: How the X Chromosome Controls Our Lives by David Bainbridge. Available in bookstores and through on-line booksellers.
Y: The Descent of Men by Steve Jones. Available in bookstores and through on-line booksellers.
Summary prepared by Nancy Novick