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