Kiecolt-Glaser et al (1984) carried out a natural experiment to investigate whether there was a difference in the immune response of participants in low and high stress conditions. They took blood samples from medical students one month before their final exams and again on the first day of their exams. When the blood samples were tested, they found that the number of Natural Killer Cells had declined and levels of Immunoglobulin A (IgA) had increased. The researchers also found that the immune response was particularly suppressed in participants who reported experiencing psychiatric symptoms, loneliness and score highly on Holmes and Rahe's Social Readjustment Rating Scale (SRRS). This study demonstrates that there is an association between stress and immunosuppression and that an accumulation of stressful events can have a negative effect on health.
A difficulty with Keicolt-Glaser et al.'s study is that, as it was a natural experiment, the independent variable (high and low stress) was not directly manipulated - it was just assumed that participants would experience greater stress on the first day of their exams compared with one month before their exams - therefore, there is a difficulty in drawing conclusions about cause and effect. Moreover, the researchers did not actually measure illness in the participants, although they did measure effects on the immune system.
A problem with research on stress and the immune system is that it is unethical to conduct experiments that deliberately expose humans to very high levels of stress. Animal experiments have, however, been conducted that demonstrate a causal link between stress and suppressed immune function.
Riley (1981), for example, used a modified turntable, rotating at 45 rpm to create stress in mice. The turntable was modified so that a whole cage of mice could be placed on it. The mice were rotated on the turntable for ten minutes per hour for five hours. During this period, Riley measured death of lymphocytes (immune cells) and degeneration of the Thymus. He found that, over time, the number of lymphocytes declined and shrinking of the thymus occurred within 24 hours. Riley observed that most of the dead lymphocytes were T cells, which destroy cancerous cells. Riley (1981) carried out further investigations by injecting normal mice with tumour cells and placing half of them on a turntable for 10 minutes per hour for 3 days, the other half acted as a control group. Riley found greater tumour growth after 30 days in the stressed mice than the control group. This shows that stress impairs the ability of the immune system to fight cancer in mice.
The studies above have demonstrated that stress suppresses the immune system in humans and animals and that in animals this suppression makes the immune system less effective at fighting illness. A study by Cohen et al. (1991) adds to this body of research by showing that people under stress are more vulnerable to the common cold.
Cohen and his colleagues (1991) assessed the stress levels of 394 healthy participants by giving them questionnaires which measured their stress levels on a scale of 1-12 and then exposed them to one of five variations of cold virus. The participants were then quarantined and monitored for the development of symptoms. They found that Infection rates ranged from approximately 74 percent to approximately 90 percent, according to levels of psychological stress, and the incidence of cold symptoms ranged from approximately 27 percent to 47 percent. Controlling for age, sex, education, allergic status and weight did not affect the results.
