Take a look at this poster. British people will probably be familiar with it already. For everyone else, it was released last year by our National Health Service when fears of a flu pandemic were at their height. When we see images of diseases and their symptoms, we typically feel disgust and repulsion. But unbeknownst to us, our immune systems have started reacting too.
In a small but compelling study, Mark Schaller from the University of British Columbia found that people who see images of sneezes and other signs of disease mount a stronger immune response to later infections than people who see unrelated images. This is the first evidence that the mere sight of a possible infection, even through a photograph, can set our bodies’ defences on high alert.
Previously, Schaller has suggested that the visual signs of disease trigger a variety of psychological tics that reduce our chances of infection. A disgusted reaction fulfils this role by making us less likely to approach potential sources of contagion. Last year, another group showed that a sneezing passer-by can make people more worried about completely unrelated threats, like heart attacks, crime and accidents. To Schaller, these reactions are all part of our “behavioural immune system” – our means of preventing infections by changing our behaviour.
But his latest study suggests that images of sickness can prime our actual immune systems too. He recruited 28 volunteers, split them into two groups, and showed them two slide shows. The first slides were just shots of furniture. The second set showed either signs of infectious diseases, such as pox, skin lesions or sneezing, or images of people brandishing guns, mostly aimed directly at the viewers.
Schaller collected blood samples from the volunteers before and after each slide show, and mixed them with molecules that give away the presence of marauding bacteria. He wanted to see how strongly the white cells in the blood would respond to these danger signs. To do that, he measured the concentrations of a protein called interleukin-6 (IL-6), which while blood cells secrete in response to infections, burns or wounds. The more IL-6 there is, the stronger the body’s immune reaction.
He found that the white blood cells responded much more aggressively to the bacterial molecules after the volunteers saw the symptom slides. The furniture images didn’t change the amount of IL-6 in the recruits’ blood samples, the gun images raised these levels by 7%, but the disease images increased them by 24%.
This suggests that the immune system reacts with extra vigour after its owner sees an image specifically related to disease, rather than one that invokes a general sense of threat or danger. Indeed, when questioned later, both groups reported the same levels of stress and fear even though only one of them manifested an actual physical reaction.
There is one caveat – the people who saw the gun images had higher levels of IL-6 in their blood samples before the experiment than those who saw the disease images. The difference wasn’t statistically significant, but it could suggest that Schaller didn’t split his groups randomly enough. He acknowledges this possibility but he says that the two groups weren’t any different in terms of their personality traits or how worried they were about disease. Nonetheless, this is an issue that could easily be addressed by doing a larger follow-up study using more volunteers.
For the moment, the effect is certainly plausible. From an evolutionary point of view, putting our immune systems on alert if we see signs of infection might reduce the odds of contracting a disease without having to distance ourselves from our social groups. However, Schaller suggests that in modern times, such responses might be counterproductive. An image of a sneeze is clearly not a sign that disease is imminent, and priming our immune systems to a non-existent threat isn’t the best use of our valuable energy.
Reference: Psychological Science http://dx.doi.org/10.1177/0956797610368064
More on immune systems:
- Smell a lady, shrug off flu – how female odours give male mice an immune boost
- Pocket Science – when enslaved bacteria go bad, gut microbes and fat mice, and stretchy beards of iron
- Genes from Chagas parasite can transfer to humans and be passed on to children
- New plant species arise from conflicts between immune system genes