Allergic diseases were almost unknown before the 19th Century. However, from the end of the 1800s there was a steady increase in the frequency of these disorders which increased sharply in the second half of the 20th Century. This increase was observed worldwide but there are large geographical differences and it is very clear that allergy has become a far more important problem for industrialised nations. 

The United Kingdom, one of the most industrialised countries in the world, was at the forefront of this rise in the main allergic conditions asthma, hay fever and atopic (‘allergic’) eczema. For example, data collected in Aberdeen, Scotland, during the 25 year interval between 1964 and 1989 shows a 2.5 fold rise in asthma and eczema and a 4 fold increase in hay fever (see left).

By 1998 allergies had become even more frequent. In that year, a worldwide survey (ISAAC Phase 1) found amongst children of 13-14 years of age that one in three suffered from asthma, one in four suffered from hay fever and one in five suffered from atopic (‘allergic’) eczema. So the UK had rapidly become one of the most affected countries in the world, alongside Australia and New Zealand all of which had allergic disease prevalence figures around ten times higher than countries in Central and Eastern Europe and in Asia.

However since this time there has been better news. The same worldwide survey repeated five years later found that the rate of increase in allergic diseases seems to have slowed down in the UK.

During the same period there was also a clear increase in other types of allergy. For example during the last decade of the 20th century, hospital admissions for anaphylaxis (extreme allergic reactions) rose seven fold, those for angioedema (allergic swellings) rose six fold and for food allergy and urticaria (hives) they doubled. Prescriptions for self-administered adrenaline injectors (a treatment for anaphylaxis) went up six fold during the same period of time.

So why has this dramatic change in the frequency of these allergic conditions occurred?

To answer this we need to consider what predisposes humans to develop allergies in the first place. One essential pre-condition is to be born with an inherited tendency for allergy. This is coded in our genes and inherited from our parents. This allergic tendency is called atopy and someone with the predisposition is known as an atopic person. However, it is certainly not guaranteed that an atopic person will always go on to become an allergic person.

So has the increase in allergy been due to a change in our inherited predisposition or from a change in our environment? The answer is fairly obvious if we consider that genes do not change easily or quickly. The abrupt rise in allergy in the last fifty years would have required an equally abrupt change in our genes. This is quite impossible, and so the only valid explanation is that the increase in allergy is caused by environmental (including life style) changes. So it is not that more people have become allergy-prone, more allergy-prone people have become allergic because factors in the environment have added together to cause it.

Until the end of the 19th century, agriculture and other outdoor occupations were the norm, houses offered very basic comfort and levels of hygiene were very poor. Allergy was rare and mostly affected people from the privileged classes.

With the advent of the industrial revolution in the 19th century, life changed dramatically and irreversibly. People abandoned agriculture in favour of industry which was expanding and offering more employment. The population became more affluent and levels of personal comfort and hygiene improved.

Housing characteristics also changed. Houses became better sealed with insulated windows and constant heating to maintain a temperature above 18-20 degrees Celsius. People mainly worked indoors and mechanisation reduced the level of physical activity. Hobbies also changed, as people abandoned physical pursuits in favour of sedentary activities.

There was also a major shift in the composition of our diet. Huge increases in the consumption of sugar, salt and commercially modified fats occurred as people replaced home prepared natural foods with commercially produced convenience foods. Major changes in transportation also occurred as cars trains and planes replaced the less efficient, but also less pollutant, animal transportation.

All these changes started at the end of the 19th century, but became more prominent in the second half of the 20th century, coinciding precisely with the period when the rise in prevalence of the allergic diseases occurred.  

Scientists are now convinced that amongst these life style changes there are allergy-inducing influences whilst, at the same time, many protective influences have been lost. Amongst many possible allergy-inducing influences the following have been closely studied.

Early exposure

The age when we first come in contact with an allergen appears to matter and the first three to six months of life appears particularly important. Contact with certain allergens during this vulnerable time seems to increase the risk for developing allergy. To give a simple example; children born in the spring or early summer, who are therefore exposed to pollen during their first three months of life, are more likely to develop hay fever when they are older.

Modern housing conditions, which promote a constant temperature and humidity, provide the ideal living conditions for house dust mites. Consequently the concentration of house dust mites in our houses is many times higher than in the past. Early exposure of infants to house dust mite allergen, added to the fact that people now spend the majority of time indoors, may explain why house dust mite has made such a major contribution to the current levels of allergic disease.

However, the relationship between exposure to other allergens and the development of allergy is not quite so straightforward. For example, studies suggest that if a child is exposed to cat allergens early in life, that child will be protected from developing cat allergy. The relationship between exposure to certain food allergens and the development of allergy also appears to be different. Allergy to peanuts, for instance, is far more frequent in the UK and USA, where peanuts are avoided by pregnant mothers and not given to young babies, than in Israel where peanuts are part of the normal diet and regarded as a weaning food for young babies. This raises the question of whether pregnant mothers and infants should or should not avoid peanuts.

Excessive hygiene

An attractive hypothesis that attempts to explain the increase in allergy was first proposed in 1989. Research had showed that allergies appeared less frequent amongst children with many brothers and sisters compared with those who had few or none. So Professor Strachan, who reported this, thought that the explanation might be that children with more brothers or sisters have more infections during childhood and that infections might protect against allergy. His findings were further explored by other researchers who found out that children growing up on a farm and those consuming unpasteurised milk have less risk of developing allergy. Strachan suggested that these children are exposed to high concentrations of bacteria and bacterial products, critical factors for priming the immune system to function efficiently.

So if infection is not always bad for us, which infection is bad and which is good? Contact with the friendly bacteria living in the gut is essential for the healthy development of the immune system. While we are in the womb, our gut does not contain any bacteria. However, this changes immediately after birth.  As the baby is born it swallows secretions from the mother and thus the first bacteria are set to colonise the gut. In the first days and months of life, the baby acquires more and more bacteria from the mother while being breastfed and also from the environment. The gut has a very active immune system, and so it would be difficult for these bacteria to go unnoticed. However, because these bacteria are not highly aggressive, they do not present us with a major risk of infection. Rather than fighting and trying to eliminate them, the immune system learns to tolerate them.

At exactly the same stage that the immune system is becoming tolerant to gut bacteria, it is also developing its tolerance of other non-harmful foreign substances such as food. Inhaled allergens, such as house dust mites and pollens, also end up within the gut because these allergens stick to the lining of the throat when they are breathed in and are then swallowed. So the same process that allows the immune system to develop tolerance of friendly gut bacteria and foods may also be important in helping us develop tolerance of inhaled allergens.

Recent studies suggest that the guts of children from developed countries who are born in conditions of strict hygiene, and who are bottle-fed from birth, do not contain the same profile of friendly bacteria as those from less developed countries and who are breast fed. This is now considered to be one of the more important environmental factors influencing the development of allergy.

So if new born babies are provided with the right gut bacteria, might they become less likely to develop allergy than those who are not? The commonest early indication for the development of allergies is eczema. Early studies have now shown that by giving certain friendly bacteria (probiotics) either as supplements to mothers during the last term of pregnancy, or to babies during first six month of life, eczema may become prevented or it may take a milder course. These are promising results, although the implications need further study before any definite recommendations can be made.

Pollution

The idea that pollution may be causing allergy to increase is not new, but its role is not quite what might have been predicted. Although pollutants are foreign substance, they do not normally cause allergic reactions directly. However their presence may cause our tissue cells to react in certain ways that make allergic reactions more likely to follow.

For example, under normal conditions the cells which form the lining of the nose and lungs, are closely stuck together by fibres that cross from one cell to another and by a glue-like substance that fills the gaps between cells. Certain pollutants may act by dissolving the fibres that bind the cells together, whilst others may act to disrupt the gluey material that binds them together. Once the surface layer is disrupted, any allergens that are present in the inhaled air can penetrate more easily into the underlying tissues where they become exposed to immune system cells.

In the case of atopic people (those born with an allergic predisposition), these immune system cells perceive these allergens to be hostile and mount an immune system response with the intention of destroying them. A consequence of this immune response is the development of unpleasant allergy symptoms in the nose and the lungs.

Certain pollutants also act by making the immune system more likely to react to otherwise harmless allergens. Many studies have shown the negative effects of pollutants on respiratory conditions such as chronic bronchitis and emphysema, and other studies have shown how they increase the risk of developing both hay fever and asthma. For example, a study of children living close to a French motorway, and thus exposed for prolonged periods to traffic pollution, showed that there was a 30% higher risk of hay fever and asthma when compared with children with a lower exposure to traffic pollutants.

However, pollution is certainly not the only factor that is responsible for the increase in allergy, and it may not even be the main factor. This has been well shown by an examination of the effects of the reunification of West and East Germany. Prior to reunification East Germany was far more polluted than West Germany but with a lower incidence of asthma and allergy. Following reunification, pollution levels in East Germany fell dramatically towards the lower pollution levels previously enjoyed by West Germany. So if pollution were the main or sole factor influencing increases in asthma and allergy, reunification would have been predicted to further reduce the low levels of asthma and allergy. However, the opposite occurred. Not only did East Germany become less polluted, its peoples were quick to adopt the more affluent dietary and other habits of the West Germans, these being likely reasons why the levels of asthma and allergy rose until they were the same across the whole country.

Eating habits

Our eating habits changed rapidly and very significantly during the twentieth century. It is not surprising then that researchers have questioned the link between these major dietary changes and the increased rate of allergy.

There are many ways that a change of diet can affect the way our immune system functions and scientists have explored a number of possibilities.

For example, in addition to the role of infant feeding methods on the balance of bacterial and other microbes that populate the gut (see above), there is also the possibility that a loss of essential nutrients from our food due to modern farming methods could contribute to similar changes. This kind of shift in the balance of gut microbes is considered to contribute significantly towards the type of immune response that favours allergy.

The variety and balance of the fats and oils we eat have also been gaining attention from medical scientists. Fats and oils are made up of various types of polyunsaturated fatty acids (PUFAs). During the last hundred years, the consumption of oils containing large amounts of omega-6 PUFAs has increased considerably. Recent studies show that these can promote abnormal responses of the immune system and may contribute to the development of allergy. Similar effects are attributed to ‘trans-saturated’ fats which were once very popular with food producers in the manufacture of margarine. This possibility has been investigated by researchers who have found a high consumption of margarine by pregnant mothers, and by children during the first years of life, associated with an increase the risk of allergic diseases.

By contrast, omega-3 PUFAs, found in high concentrations in oily fish, linseed oil, rape seed oil, soya and walnut, appear to have an allergy protective effect. Infants considered especially ‘at risk’ for developing allergic diseases appeared to have this risk significantly reduced by the introduction of omega-3 PUFAs either to the mother’s diet towards the end of pregnancy or the infant’s diet during in the first 1-2 years. There is also evidence that children who regularly consume oily fish suffer less asthma and wheezing.

Modern medicines

The beneficial effects of modern medicines on human health are now taken for granted. Certain drugs and medicines have contributed very significantly towards a reduction in many diseases. However, it would be wrong to assume that this benefit has occurred without any risk that other diseases may have been adversely affected by these same drugs and medicines. The explosion in use of prescription drugs has been accompanied by a similar increase in the use of popular medicines. 

Two common medicines, antibiotics and paracetamol, are worth considering further here.

Antibiotics, particularly when taken during the first year of life, appear to increase the risk of both allergic eczema and asthma. A recently published study of German children, examined those treated with one of two types of antibiotic (cephalosporines and macrolides) given for respiratory infections encountered during the first year of life. Because many doctors do not consider it necessary to prescribe antibiotics for uncomplicated viral respiratory infections, the infant patients of these doctors formed an untreated control group as a comparison. The study found that there was a trebling of the risk of developing allergic eczema in the antibiotic treated infants in comparison to infants that did not receive these antibiotics.

Similarly, children treated with antibiotics during their first year of life had a 4-fold higher risk of developing asthma. Children who received antibiotics only in their second year of life suffered a smaller risk, but this was still one and a half times the risk in children who were not given an antibiotic.

It is still not entirely clear why antibiotics increase the risk for allergy, although the explanation may be that antibiotics destroy not only ‘bad’ bacteria; i.e. those that were causing the disease for which they were given, but also the ‘good’ bacteria in the gut. These bacteria have been shown to be essential for efficient immune system function, and especially the mechanism that permits tolerance of the harmless foodstuffs that we eat. So when these ‘good’ gut bacteria are replaced by other bacteria or fungi, the immune system enters a state of alertness and hyper-reactivity. Its ability to distinguish between what is harmful and what is harmless is blunted and it begins to react to food allergens such as nuts, seeds and certain fruits and to airborne allergens such as pollens and dust mites.

Paracetamol may also increase the risk for allergy. Recent studies have found that it raises the risk for asthma and wheezing and it has also been linked with hay fever and eczema.

Dr Diana Church 2010