The researchers evaluated 30 male volunteers, who were exposed to two different types of stressful situations: an ‘active’ stressful event, where the volunteers were asked to memorise information and take a 12-minute test; and a ‘passive’ stressful event, where volunteers watched a 12-minute video of ‘gruesome’ surgical procedures.

During the experiment, each participant gave saliva samples that were analysed for immune system components known as secretory proteins. These proteins help to protect the lining of organs such as the lungs and stomach to guard against the invasion of bacteria and viruses.

While levels of secretory proteins increased during active stress, the immune response dropped during passive stress.

Most acute stressors boost the immune system, at least initially. It is only when stress is unusually prolonged or repetitive that it becomes a potential threat to health. An exception to this rule appears to be when we experience stressors for which the only response is to passively endure. These types of stressful situations, which people might describe with the phrase 'my heart stopped beating', have a rapid and strong suppressive effect on some aspects of immune function.

Bungee jumpers, parachutists and other thrillseekers will be delighted to hear that the authors of this study recommend these kinds of challenges as active stress and, therefore, beneficial to the immune system and your overall health (Psychophysiology, 2001; 38: 836-46).