One of the newest and most exciting fields of biological research at present concerns the relationships between the immune system, the nervous system and the hormones. The realization that these three systems can interact, and the identification of the mechanisms involved, has only come within the past few years. This new science has been given the rather daunting name of psychoneuroimmunology.
Among the discoveries made in psychoneuroimmunology is that stressful events can make the immune cells far less responsive to infection. Bereavement can have devastating effects on our defensive cells, but even something as minor as taking an exam can make us more vulnerable to infection. With long-term stress, it appears that a sense of being in control makes all the difference – feelings of helplessness and inability to improve matters are the most damaging. Controllable stress, on the other hand, can actually improve immune status.
The mechanisms behind these interactions are still waiting to be unravelled, but there are indications that small messenger molecules may be important. These small messengers include the major chemical signals produced by the body (hormones), messenger substances released by the nerves for communication with adjacent nerves (neurotransmitters) and mediators released by immune cells which stimulate or suppress other immune cells (lymphokines). Some of the hormones known to affect the nervous system, such as the endorphins, now appear to bind to immune cells as well, and probably influence their behaviour. Conversely, mediators produced by immune cells can influence nerve cells – histamine and prostaglandins both have this effect. One lymphokine with marked effects on both body and mind is interferon. The hormone, noradrenaline, also acts as a neurotransmitter for some nerves. In other words, these are three closely interconnected systems.
There are also direct links between the nerves and the immune system that do not rely on messenger substances. Detailed anatomical studies have revealed nervous connections that were not previously suspected. It turns out that several parts of the immune system – including the lymph nodes, the spleen, the thymus gland and the bone marrow – are connected by nerve fibres to the central nervous system. Exactly what effect the nerves have on these organs is as yet unknown.
So far, research into psychoneuroimmunology has done no more than scratch the surface of this potentially important topic. But it indicates that the idea of allergies affecting the mind – and vice versa – is not implausible.