Is being too clean responsible for the rise in allergies?
It’s often claimed that we’re living “too clean a life” these days, and this is one reason why allergy rates have risen. Chris Smith put this to Sheena Cruickshank from the University of Manchester...
Sheena - What you’re describing there is something call the Hygiene Hypothesis which was developed a few years ago by a scientist called David Strachan. And what he observed was that people who lived in the country and had very big family groups were much less likely to have allergies than people who lived say in the city and had quite small families. His idea was perhaps the difference between the sort of large family group and the country was that you go a lot more exposure to a whole variety of different germs at an early age, which then trained your immune response up so that it was much less likely to misfire. But this hypothesis has very slightly gone out of favour now. We have a new version called the Old Friends hypothesis. Do you want me to tell you about that?
Chris - Go on then tell me about some of my old friends.
Sheena - Your old friends. Well, this is taking the idea that, obviously, we’ve got all sorts of things that live inside us and on us. We have the bacteria and the viruses etc., that make up our microbiome that are very very important for our health. And these develop in response to things that we eat, things that we’re exposed to in the environment - all sorts of things, and perhaps they are one of the factors that could be really important in helping develop our immune response. Then there’s the other idea as well that we don’t get the same types of infections that we used to. If you go back to the time of King Richard III, we know that we all used to have a lot of parasitic worm infections which is what Eoin was talking about, and our immune response therefore evolved alongside these worm infections. We don’t have these now, so is it the combination of lacking our old friends - things like the worms, or having very different types of bacteria that we breathe in or live in and on us that means that our immune response doesn’t get the right signals to develop and then goes wonky?
Chris - There’s also evidence that the microbes that live in our mum’s gut when we’re developing inside our mum have a chemical conversation with the developing baby in terms of patterning how the blood/brain barrier that separates the brain tissue from the bloodstream develops. So do you think there’s also a possibility that what mothers eat when they’re pregnant could affect the susceptibility of their offspring to being allergic?
Sheena - It’s possible. I think there’s evidence for and against that. There were some studies that were sort of suggesting that you shouldn’t eat peanuts, for example, when you’re pregnant because it would be bad for the baby, and then a lot of the allergies actually went up. So I think we don’t know, but it’s very very clear that the bacteria that we have are very important for the development and education of our immune system and if you’re born in a very sterile environment - so if you take a mouse that never has any bacteria - it’s immune system doesn’t develop as well as one that is exposed to bacteria. And, certainly, we do get exposure through the maternal interface with their bacteria and also we get education from the immune system, so there might be something in that. I think there’s a lot more that we need to find out.
Chris - So Sheena, does this mean then that perhaps we should be encouraging people in the modern era to embrace the microbial world more, go and roll around in dirt and muck and allow our kids to go and get more filthy than we do or actually, probably is that not such a good idea?
Sheena - Well, I think the most important thing to realise is that until we had the sort of hygienic conditions that we do and particular changes in sanitation, our average lifespan was around the age of 40 and so, basically, being cleaner has made us able to live a lot longer. So I think, overall, being clean is better than not being clean and having less infections is better for you.