The main function of the immune system is to protect the body from infectious agents such as viruses and other toxins. The immune system can fail us in two ways-either by, letting infections enter the body, or being over-active, so that it is the immune system itself, rather than an infectious agent that causes illness. Most studies of the relationship between stress and the immune system have focussed on acute stressors and have found a decrease in immune cell function. One study which relates to stress and the immune system was Cohan (1993). Cohen investigated the role of general life stress on vulnerability to the common cold virus. Three hundred and ninety-four participants completed questionnaires on the number of stressful life events they had experienced in the previous year. They also rated their degree of stress and their level of negative emotions such as depression.
The three scores were combined into what Cohen called a stress index. The participants were then exposed to the common cold virus, leading to 82% becoming infected with the virus. . The findings were that the chance of developing a cold was significantly linked with stress index scores. A strength of this study was that it did measure health outcomes, showing a relationship between life stress and illness. This can be compared with studies that use measures of immune function rather than illness outcomes. Also, this was an indirect study in the sense that there were no direct measures of immune function.
However it is supported by Evans and Edgington (1991) who found that the probability of developing a cold was significantly correlated with negative events in the preceding days. However there are many limitations. During the study participants should be constantly monitored to check for any reactions to the viral challenge which had affected their health and the scientific value of the study should be balanced against any psychological or physical distress to participants. Another study which looks at the link between stress and the immune system was a study carried out by Brady in 1958. He linked high levels of stress to increased hormone production and the development of ulcers. In an early study he placed monkeys in “restraining chairs” and conditioned them to press a lever. They were given shocks every 20 seconds unless the lever was pressed in the same time period.
This investigation came to an abrupt halt when many of the monkeys suddenly died. After 23 days of 6 hours on, 6 hours off schedule the executive monkey died due to a perforated ulcer. He tried various routines, such as 18 hours on and 6 hours off, or 30 minutes on, 30 minutes off. However, no monkeys died from ulcers. He then tested the stomachs of executive monkeys on a 6 hour on, 6 hour off schedule, and found that stomach acidity was greatest during the rest period. Brady concluded that it was clearly stress, not the shocks that created the ulcers. One criticism made of Brady’s study in general was that the monkeys were not randomly selected, the “executive” was chosen because it was faster at learning an avoidance response.
This may of course have parallels with the human world. Also, the fact that the study was carried out on monkeys means that the results cannot be generalised for humans. Also another limitation was that a more serious problem was raised in the research by Marshall et al (1985). They found strong evidence of another cause of stomach ulcers, a bacterium called Helicobacter pylori not stress related.