Recall the story of Selye and his ulcerated rats. There was brief mention of shrunken thymus gland. It turns out that the best documented way in which immune suppression occurs is via the glucocorticoids. These stress-response hormones have a pretty wide area of influence in the immune system. They cause the shrinkage of the thymus, inhibit the release of the messengers interleukin and interferons and they make circulating lymphocytes less sensitive to infectious alarms.
The glucocorticoids cause the lymphocytes to be pulled out of circulation and even worse can actually kill them. Some other stress-response hormones also suppress immunity (like beta-endorphins) though their role is far from clear and the effect is far less than that of glucocorticoids.
The alert reader at this point may ask why the immune system is suppressed during stress? All along, we have been emphasizing that the body’s stress response has evolved superbly to fight physical stressors. So, why has evolution favoured a system that seems to knock out the immune system when faced with a stressor? Logically, it does not make sense to lower your guard when faced with a stressor.
In fact, it seems the opposite should be true. One of the things we noted was that during a stressor, long-term tasks that involve expenditure of energy are shut off (digestion, ovulation and so on). In that light, consider what the body does to the immune system—it expends energy to actively dismantle the immune system when it ought to be spending energy to shore up the defences.
The story of the immune system and stress is actually pretty complicated. Newer techniques, particularly in developing extremely sensitive tests to determine levels of hormones have led to a nuanced picture of what happens to the immune system in response to stress. First, as expected, the immune system is enhanced at the onset of stress. For the first 30 minutes or so of a stressor, the immune system is enhanced. After about 60 minutes, with sustained release of glucocorticoids and the workings of the sympathetic system, the immune system starts to be suppressed.
If the stressor is of moderate duration, the net effect is that the immune system is brought back to the pre-stress level. No harm done. It is only in the face of chronic stress that the suppression of the immune system is such that it goes below the pre-stress level. Again, it reinforces the major theme of this book. It is the chronic stress and the body’s response to it that causes the problems. Alert readers will ask as to why the immune system should be brought down from the high level it had reached during stress?
Is it not good for the body to have the immune system working in top gear? The first obvious answer is that it would be too costly—the effort required to keep the immune system at an enhanced level will require a considerable expenditure of energy.5 The second explanation is that a chronically activated immune system becomes even more active, spiraling out of control and leading to autoimmune diseases.
To summarize—
• During stress, the stress-response hormones cause a transient activation of the immune system.
• Stress response enhances the immune defences and helps in redistributing them to the site of the infection.
• To avoid the danger of overshooting into autoimmunity, exposure to glucocorticoids brings the immune system back to the pre-stress level.
In the case of chronic stress and prolonged glucocorticoids release, the immune system is suppressed below normal levels. These findings help to explain some facets of the autoimmune disease and increased vulnerability to infections as will be discussed in subsequent sections.