Could hygiene obsession cause allergies?
Is the modern world's obsession with hygiene responsible for a global allergy epidemic? Dan Cossins investigates.
An epidemic is sweeping the Western world. The symptoms vary: sneezing, rashes, swollen eyes and breathlessness.
The illness is often debilitating and can be fatal. But the causes are seemingly harmless things like pollen and peanuts. This isn’t some fictional doomsday scenario. This is the here and now, where an inexorable rise in allergies is emerging as a serious threat to the health of future generations.
A century ago, no such problems existed. Now, in some parts of the world, allergies affect a third of adults and almost half of children, and they’re becoming more severe. If this continues, allergy experts warn that it could become one of the biggest medical challenges of the next century. So why are our bodies freaking out so much?
It has long been suspected that our hygiene-obsessed modern lifestyle is to blame. But immunologists are now revealing precisely how reduced exposure to bacteria and parasitic worms is damaging the immune system’s ability to regulate itself. Immune responses that evolved in the presence of certain microbes have been thrown wildly out of kilter in their absence. As a result, the body’s defences have become dangerously over-sensitive to things like dust, pets and foods.
And it’s not just allergies. “The same malfunction is also responsible for various other immune-related diseases, including 3 3 type 1 diabetes, inflammatory bowel disorders and multiple sclerosis,” says Guy Delespesse, director of the Lab for Allergy Research at Montreal University. “It is a major problem for us.”
The incidence of these ailments has risen exponentially of late. In 1980, just 10 per cent of the Western population suffered from allergies. Today, it’s more than three times that. And, according to predictions from the Global Allergy and Asthma Network, by 2015 half of us will suffer from allergies. A group of leading experts recently claimed to The Times that we’re now “in the midst of an allergy epidemic”.
So what’s to blame? Allergies have a strong inherited component, but their rapid rise and peculiar distribution suggest environmental factors. While rare in developing countries, in the West allergies are much more common in cities than rural areas. “All the epidemiological evidence suggests that the rapid modification of the environment in industrialised countries must be in some way responsible for this dramatic rise,” says Delespesse. “The question is how.”
Several explanations have already been proposed, from increased exposure to allergy triggers in carpeted homes and polluted cities to changes in diet, obesity and breastfeeding patterns.
The theory that gained most credence, however, was the ‘hygiene hypothesis’, first proposed in 1989. Observing that allergies were less frequent among people who had serious illnesses during childhood, David Strachan, an epidemiologist at St George’s Hospital Medical School, London, argued that modern immune systems were ill-prepared due to the reduction in serious childhood infection. The idea was that if you haven’t suffered those infections, you don’t have enough Th1 response cells, one form of immune attack cells, which upsets the required balance and renders another form, the Th2 cells, trigger-happy.
In other words, in the confines of our antiseptic modern lifestyle, the immune system falls idle, fails to mature properly and attacks harmless foreign molecules, creating all sorts of self-destructive reactions.
Then, in 1995, the discovery of regulatory T-cells, specialist cells that function exclusively as a brake on the attack cells, revolutionised the way we view the body’s defences.
“Instead of doing nothing until provoked, the immune system is constantly having to be restrained and these regulatory T-cells control that,” explains Dr Graham Rook, Professor of Medical Microbiology at University College London. “What’s actually happening with allergies is a problem of immuno-regulation.”
Several studies have since shown that it’s actually exposure to harmless micro-organisms (or ‘old friends’ as Rook calls them), rather than harmful diseases, that determines how well the immune system regulates its highly aggressive nature.
Rook has shown that mice with respiratory allergies get better when treated with dead Mycobacterium vaccae, a harmless bacterium found in mud. Such studies also suggest that the shift to an antibacterial lifestyle may have caused this disruption of immune development, isolating the body from those friendly microbes.
“The micro-organisms that we evolved with, long before we began this modern lifestyle, became a crucial part of our physiology,” says Rook. “In this state of ‘evolved dependency’, these microbes took the role of switching on the regulatory pathways that allow our immune systems to function as they should. Without exposure to these microbes, our immune system attacks otherwise harmless foreign molecules.” Rook has labelled this new hypothesis the ‘old friends mechanism’; others call it the ‘microbial exposure theory’.
Whatever it’s called, the next step is to identify which microbes are responsible for this all-important schooling of the immune system. To that end, immunologists are focusing on the microbes that humans encountered on a daily basis back when we drank from streams and toiled among soil and farm animals. It was in their company that our defence mechanisms evolved, after all.
A series of studies carried out by Dr Erika von Mutius, head of the Asthma and Allergy department at Munich University, have shown that children who grow up on farms have a far smaller chance of developing allergies than kids growing up in urban areas. “The farm 3 3 studies show that there must be some combination of factors, some probably microbial – whether from animal faeces, grass dust or unpasteurised milk – that guard against immune regulation problems,” says Rook. “But it’s very difficult to isolate the ones that make the most effective contribution.”
Meanwhile, Dr Bengt Bjorksten, Professor of Paediatrics and Allergy Prevention at Stockholm’s Karolinska Institute, is focusing on the gut, where a vast surface area of intestinal wall plays host to an astonishing diversity of microbial life.
Comparing the gut microbes in babies from Sweden, where allergy rates are high, and those from neighbouring Estonia, where living conditions are less sanitised and allergy rates are low, Bjorksten has revealed interesting changes in our gut ecology. Estonian babies were colonised more rapidly and by a greater range of microbes than their Swedish counterparts, suggesting this diversity is a key factor in allergy resistance.
Many scientists are convinced that effective treatments are not far away. Immuno-biologists both here and in America are exploring the effect of helminths – otherwise known as parasitic hookworms – on allergic responses in humans, and the early clinical trials are providing encouraging results. Meanwhile, a recent German study – in which mice exposed to barnyard microbes gave birth to allergy-resistant offspring – indicates that exposing pregnant women to such microbes may protect their children.
Elsewhere, allergy experts like Guy Delespesse recommend probiotics, in the form of yogurt drinks, as a way of smuggling the good bacteria into the body. And most agree that hygiene needn’t be so absolute. “We’re not saying abandon it altogether,” says Rook, “but obsessive attention to the wrong kinds of hygiene should be curbed. If a kid comes home with muddy hands, it’s unlikely to do any harm. And may well do some good.”
Experts are optimistic about the discovery of the immunological elixir that lurks on the farms and in the guts of allergy-resistant people across the developing world. It should then be possible to find ways to re-educate our frontline troops in the complex art of immunological warfare.
“It’s going to be complicated to isolate the active molecules and transform them into treatments,” concludes Rook. “But I think we’re on the right track. There is hope for future generations.”