An allergy is when the body’s immune system is sensitive to a normally harmless molecule. In most people, this molecule would have no effect, but in those with an allergy, the body sees it as a threat and reacts abnormally to its presence.
Allergies are becoming more and more common in the Western world. The number of children with a food allergy has doubled in recent years, and the World Allergy Organisation recently revealed that the global prevalence of asthma (a common symptom of allergies) has increased by 50 per cent every decade for the past 40 years. 50 years ago, one in 5000 people were allergic to wheat; this figure is now closer to one in 130. Allergies are becoming a huge problem, with 50% of children in the UK having an allergy, and 20,000 people admitted to hospital each year for a dangerous (and potentially life-threatening) allergic reaction.
Scientists disagree about the explanations for this increase. Theories include genetic reasons, a change in diet, and something called the ‘hygiene hypothesis’. The idea behind this is that allergies are so common now because we are kept too clean as children, when our immune systems are developing. This means that the immune system is not exposed to as many pathogens, so cannot regulate itself as well. The basic theory was proposed in 1989 by Strachan, who said that young children exposed to infectious diseases will be less likely to suffer from allergies. Since then, it has been developed and is now also known as the “lost friends hypothesis”. It is believed that, as well as colds, measles and other common childhood infections (which have only evolved in the last 10, 000 years), it is exposure to ancient microbes present in the time of human evolution that can prevent allergies; we have “lost” our “old friends”, whom our immune systems need to develop properly.
Although the hygiene hypothesis has not been scientifically proven, there is lots of evidence that supports it. Links exist between increased allergy prevalence and many factors related to cleanliness (e.g. early day care attendance, rural living, contact with animals, older siblings, large family size, and infection by common diseases).
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It makes sense that a child in day care will have increased exposure to infections. In fact, many parents send their child to day care so they will become immune to diseases such as chicken pox, which can be dangerous if caught later in life. It has been found that children who went to a large day care with other many children have a reduced likelihood of developing an allergy.
Several studies report a reduced incidence of hayfever and asthma in the children of farmers. In particular, factors to thank for this are: contact with animals as a child, exposure to stables under the age of one, and consumption of farm milk (presumably raw/ unpasteurized). As farm animals can be considered ‘dirty’, this suggests that exposure to common farmyard microbes may influence vulnerability to allergies. One of the most significant links with allergy prevalence is family size. This is because being in a larger family, with more children, means more microbes and infections are brought into the home. Hay fever and eczema are less common in larger families, and a study on asthma showed that being from a small family increases the chances of a child being diagnosed. It has been found that having many older siblings (at least three) in particular can have a protective effect from allergies. Sharing a bedroom as a child, which is more likely in large families, also had a protective effect. This all agrees with the hygiene hypothesis, as this would provide more opportunity for exposure to microbes or infection.
The huge increase in allergy prevalence has been seen much more dramatically in the industrialised world than in developing countries. This could be for genetic reasons, but there is evidence that this too is due to different levels of exposure to microbes and disease. Firstly, it is known that the average Eastern family is larger than the average Western family, which, as we know, decreases the likelihood of developing an allergy. Furthermore, immigrants from developing countries have been found to increasingly develop autoimmune disorders in relation to the length of time they have been in the industrialised country. Studies in Ghana demonstrate an increase in immunological disorders as it grew more affluent and presumably cleaner.
Allergies are on the rise, and it seems that the increased hygiene in the Western world may be the cause.