A wide variety of maladies, including stomach upset, hives, and even heart disease, are linked to the effects of emotional stress. But although the relationship between stress and immune function is being studied by a number of different types of scientists, so far it is not a major area of research for immunologists.
Studying the relationship between stress and the immune system presents difficult challenges. For one thing, stress is difficult to define. What may appear to be a stressful situation for one person is not for another. When people are exposed to situations they regard as stressful, it is difficult for them to measure how much stress they feel, and difficult for the scientist to know if a person’s subjective impression of the amount of stress is accurate. The scientist can only measure things that may reflect stress, such as the number of times the heart beats each minute, but such measures also may reflect other factors.
Most scientists studying the relationship of stress and immune function, however, do not study a sudden, short-lived stressor; rather, they try to study more constant and frequent stressors known as chronic stress, such as that caused by relationships with family, friends, and co-workers, or sustained challenges to perform well at one’s work. Some scientists are investigating whether ongoing stress takes a toll on the immune system.
But it is hard to perform what scientists call “controlled experiments” in human beings. In a controlled experiment, the scientist can change one and only one factor, such as the amount of a particular chemical, and then measure the effect of that change on some other measurable phenomenon, such as the amount of antibodies produced by a particular type of immune system cell when it is exposed to the chemical. In a living animal, and especially in a human being, that kind of control is just not possible, since there are so many other things happening to the animal or person at the time that measurements are being taken.
Despite these inevitable difficulties in measuring the relationship of stress to immunity, scientists who repeat the same experiment many times with many different animals or human beings, and who get the same result most of the time, hope that they can draw reasonable conclusions.
Some researchers place animals into stressful situations, such as being trapped in a small space or being placed near an aggressive animal. Different functions of their immune systems, and their health, are then measured under such stressful conditions. On the basis of such experiments, some published studies have made the following claims:
- Experimentally created “stressful” situations delayed the production of antibodies in mice infected with influenza virus and suppressed the activity of T cells in animals inoculated with herpes simplex virus.
- Social stress can be even more damaging than physical stress. For example, some mice were put into a cage with a highly aggressive mouse two hours a day for six days and repeatedly threatened, but not injured, by the aggressive mouse — a “social stress.” Other mice were kept in tiny cages without food and water for long periods — a “physical stress.” Both groups of mice were exposed to a bacterial toxin, and the socially stressed animals were twice as likely to die.
- Isolation can also suppress immune function. Infant monkeys separated from their mothers, especially if they are caged alone rather than in groups, generate fewer lymphocytes in response to antigens and fewer antibodies in response to viruses.
Excerpted fromThe Truth About Your Immune System, a Special Health Report from Harvard Health Publications