Personality of the immune system

Our personality literally shapes our world. It helps determine how many friends we have, which jobs we excel in and how we cope with adversity. Now it seems it may even play a role in our health – and not just in terms of any hypochondriac tendencies we harbour, but also how prone our bodies are to getting sick in the first place. It is a provocative idea but one that has been steadily gaining traction.

We think of conscientiousness, for example, as a positive trait because it suggests caution, careful planning and an aversion to potential danger. But could it also be a symptom of underlying weakness in the immune system?

That’s one interpretation of a study published last month that sought to pick apart the links between personality traits and the immune system. It found that highly conscientious people had lower levels of inflammation; an immune response that helps the body fight infection and recover from injury. Highly extrovert people had higher levels.

This may mean that extroverts are more physically robust – at least while they’re young. While this sounds like good news, there’s also a downside since sustained inflammation over a lifetime may leave you vulnerable to diabetes, atherosclerosis and cancer.

Past studies have hinted at a link between personality and the immune system. However, the current one is the first to assess personality across a range of people, and connect it to the activity of a suite of genes that control how the immune system works. “They are looking at the immune system in a much more cohesive and comprehensive way,” says a professor of biological psychiatry at King’s College London.

In the latest study at the University of Nottingham, UK, 121 healthy students filled in personality questionnaires to assess the so-called big five personality traits – conscientiousness, extraversion, neuroticism, openness and agreeableness. They also asked them about other behaviours such as smoking, drinking or exercise that might be associated with certain personality types. They took blood samples to assess the activity of 19 genes involved in the inflammatory response, as well as genes involved in the production of antibodies and viral defence.

Even after controlling for behaviour such as alcohol consumption, they found that on average, the genes that trigger inflammation are 17 per cent more active in extroverts than in introverts. In students who scored high for conscientiousness the activation of these genes was 16 per cent lower compared with less conscientious people. There was no apparent difference in the other immune system genes

“The idea that a huge amount of who and what we are is influenced by the way our species fights disease is a powerful and emerging theme across lots of different areas,” says Daniel Davis, an immunologist at the University of Manchester, UK. “A link with personality is not outside the world of what should be looked at,” he adds. However, he cautions that it will take more work to confirm the extraversion, conscientiousness and inflammation link. And if it does exist, do the observed differences translate into meaningful differences in health?

Assuming it does hold, the next question is whether the immune system shapes personality, or vice versa. We know that factors such as stress can boost the activity of inflammatory genes, triggering a short-term boost against infection. Conscientious people might have less inflammation simply because they take better care of themselves than extroverts do – so are less likely to get injured or be around other sick people who could potentially pass on germs. It would be a terrific idea to boost inflammation genes if you’re running around meeting new people, says Cole.

However, it’s also becoming clear that the immune system can influence your mood. A good example of this is “sickness behaviour”; the tendency to become lethargic and withdrawn in response to infection. Possibly, it’s a two-way street. “The mechanisms don’t have to be mutually exclusive,” says Damian Murray, a psychologist at UCLA.

So how might the immune system influence behaviour? In the short term, immune cells release chemicals called cytokines that seem able to cross the blood-brain barrier and interfere with brain cell activity. For example, the release of gamma interferon reduces serotonin production and makes people feel less sociable.

But some think the immune system might also affect the evolution of personality traits like conscientiousness. “In the course of evolution, humans have faced adaptive challenges caused by infectious diseases,” says Napolioni at the Stanford University School of Medicine. “In addition to the immune system, human behaviour may also act as an anti-pathogen defence [by enhancing the survival of people with weaker immune systems].” Last year, Napolioni showed that Americans who carry a variant of the ACP1gene, which boosts susceptibility to infections, are more introverted and less open to new experiences.

Even if conscientious and introverted people do have weaker inflammatory responses, other areas of their immune defence may be stronger, says Davis. As for extroverts, if they really are at greater risk of diseases linked to inflammation, exercise and weight loss can help.

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