The implications of psychoneuroimmunology cover the length and breadth of medical research.
Over the last few decades, the intriguing and pervasive links between neuroscience and the immune system have slowly been uncovered.
What might seem, at first, like an uneasy marriage between the brain and immunity has steadily grown into a fully fledged interdisciplinary area of study.
This field is known as psychoneuroimmunology (PNI).
It is well established, in the minds of most people, that stress can induce illness and that, conversely, a fun-filled occasion with loved ones can soothe aches and pains and stave off the very same illness.
What might have been referred to as pseudoscience a few decades ago now finds strong support from many quarters. PNI has deep ramifications for the future of medical research, the treatment of diseases and our attitude toward handling stress.
In this article, we will take a look at the birth of PNI, how the immune and nervous systems interact and some of the ways in which these communication pathways affect us all.
The mind's impact on health
First, we will take a very brief look at a few examples of how psychology has been shown to influence the immune system:
- Bereavement: stories of recently bereaved individuals dying soon after their partner are common. These tales are not just apocryphal. A study that followed 95,647 recently widowed individuals found that during the first week after bereavement, mortality was twice the expected rate. There is more to this than a metaphorical "broken heart"
- The gut: it is now fairly well established that there is a strong association between sustained stressful life events and the onset of symptoms in functional gastrointestinal disorders, inflammatory bowel disease and irritable bowel syndrome
- Cancer: health professionals working with cancer patients know only too well that a patient's outlook and their quantity and quality of psychological support can hugely impact the outcome of their disease
- HIV (human immunodeficiency virus): studies have found significant evidence that elevated levels of stress and diminished social support accelerates the progression of HIV infection
- Skin complaints: psoriasis, eczema and asthma are all known to have psychological aspects to them. A stressful day at the office can have you scratching as you reach for the asthma pump
- Wound healing: the speed at which a surgical patient heals has been linked to psychological factors. For instance, increased levels of fear or distress before surgery have been associated with worse outcomes, including longer stays in the hospital, more postoperative complications and higher rates of re-hospitalization. In one study on patients with chronic lower leg wounds, those who reported the highest levels of depression and anxiety showed significantly delayed healing.
Despite first-hand accounts of stressful or exhausting psychological events negatively impacting physical well-being, the scientific evidence behind these stories was not initially forthcoming.
How could neural activity influence the activity of the immune system? The immune system's classical messaging system - the lymph system - is not present in the central nervous system, so conversations between the two were considered impossible.
What sounds like medieval quackery is now considered science fact; the mechanisms that underpin immune-brain interactions are steadily being uncovered.
As with so many scientific discoveries, it was a chance observation that got the ball rolling.
The birth of psychoneuroimmunology
Robert Ader is widely considered to be the father of modern PNI. His early research, involving conditioning in rats, opened the floodgates for the study of brain-immune communication.
Experiments into psychological conditioning accidentally stumbled upon the brain-immune interaction.
Ader, a psychologist by trade, worked closely with Nicholas Cohen, an immunologist.
Their specialties made them the perfect team for the job, even though they did not realize it at the time.
Their landmark discovery was courtesy of science's old friend - serendipity.
Ader was working on variations of the classic Pavlov's dogs experiment: salivation in dogs was conditioned by an auditory stimulus - such as a metronome - before they were fed each day. Consequently, the stimulus induced salivation without the presence of food.
In Ader's version of the experiment, he fed rats different quantities of saccharin solution and simultaneously injected them with Cytoxan - a drug that induces gastrointestinal distress and suppresses the immune system. The rats were conditioned to avoid drinking the solution, as predicted.
Ader then ceased injecting the rats but continued to present the saccharin-laced water. The rats avoided the solution but, strangely, some of them died. He noted that the avoidance response and the level of mortality varied depending on the amount of saccharine water they had been presented with.
The results intrigued Ader; it seemed that the avoidance response had been conditioned as expected, but, unexpectedly, so had the corresponding drop in immunity. In an interview in 2010, he explained:
"As a psychologist, I was unaware that there were no connections between the brain and the immune system, so I was free to consider any possibility that might explain this orderly relationship between the magnitude of the conditioned response and the rate of mortality.
A hypothesis that seemed reasonable to me was that, in addition to conditioning the avoidance response, we were conditioning the immunosuppressive effects [of Cytoxan]."
His next study, published in 1975, proved beyond doubt that his hunch, although surprising and openly mocked by other scientists, was spot on.
The game truly had changed. A neural signal (taste) had managed to trigger a conditioned reduction in the immune system. The results were replicable, and although the theory received more than its fair share of flack, there seemed no other way to explain it.
All of a sudden, the central nervous system and immunity were bedfellows.
On the next page, we look at the mounting evidence for brain-immune interactions, the role of neuropeptides and how the brain communicates with the immune system.