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
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
Allergies are on the rise, and it seems that the increased hygiene in the Western
world may be the cause.