Is allergy more common today?
Asthmatic army conscripts
During times of crisis, a country's very survival may depend on its soldiers. But conscripting young men to perform national service in the army offers some non-military opportunities. For example, it can provide an insight into changes in disease incidences, for example it may help answer the question – is asthma more common today? The army keeps meticulous records of the health of the conscripts. In Finland, 98 % of all men undergo a medical examination at the age of 19 to establish their fitness for military service. Subsequently the army monitors their condition during their entire period of military service.
A group of researchers has started to mine this seam of health data dating back to the 1920s and the results obtained have been startling. It seems that asthma was an extremely rare condition in the 1920s, less than 0.1 % of conscripts were diagnosed as being asthmatic in that decade. In 1961, the condition had not changed to any extent, in that year a mere 0.08 % of conscripts were diagnosed with asthma. However, from that time on, asthma has become more and more common; by 1989 its incidence had risen to 1.79 % (an increase of over twenty-fold from the 1920s). At the same time, the number of conscripts diagnosed with asthma during their time in the army and also those discharged because of asthma also exhibited a marked increase. It is possible that the diagnostic criteria have become less rigorous during this time period, but this is unlikely to account for a twenty-fold change.
Allergy – an indirect index of the standard of living?
All types of asthma are not due to allergy, neither do all patients with allergies invariably become asthmatics. Have all types of allergy increased? Answers are starting to emerge from several countries. Allergies tend to be associated with an urban lifestyle. They tend to occur in better-paid, upper and middle-class families and are associated with good hygiene. This is nicely illustrated by examining two geographically adjacent but economically quite different communities, one in North Karelia in Finland just across the Russia-Finland border from the community of Pitkäranta. The Finns are more than twice as likely to suffer allergies as their Russian neighbours. A similar difference has also been documented between communities in the east and west parts of the old internal border through Germany. After unification, the difference in the incidence of allergies is disappearing.
One possible explanation, especially with respect to asthma, is the degree of air pollution and this has been investigated in the two halves of Germany. It appears that the conventional pollutants, such as sulphur dioxide, cannot account for the difference. The air over East Germany was more polluted but West Germans were more likely to suffer from asthma. It is still possible that more specific pollutants such as the small diesel exhaust particles emitted by cars and buses can explain this paradox.
There are reasons to suspect that microbiological environmental factors may account for the inexorable rise in the incidence of asthma – the so-called hygiene hypothesis. Researchers have noted that the eldest child in a family (or the only child in one-child families) is more likely to suffer allergies than younger siblings.
Th2 immunity – immunity targeting parasites
There are many different types of lymphocytes which belong to the white blood cells. One sub-group, the T-cells, possesses the ability to recognize foreign materials and to create a memory of this exposure. Subsequently, these cells act as judge and jury in deciding whether something is recognized as "own" or "foreign" – in case of the latter, it will be tagged for destruction. There are T-helper cells Th1 and Th2 which possess slightly different properties. Th1 cells trigger activity down a cell-mediated pathway whereas the Th2 cells activate the antibody-mediated system. Thus, both systems are intended to protect the body from foreign material but their mechanisms are very different.
The type of immune response triggered in allergic and atopic individuals is Th2 mediated. This is the system which is intended to be activated in response to attack by parasites. Afro-Americans appear to be endowed with an immune system appropriate to their African origins. However, in the United States, these individuals are not exposed to the same array of parasites in their natural environment as they would be if they were living in the tropical rain-forests of equatorial Africa. This may explain the higher incidence of allergies and atopy in Afro-Americans, their immune system is designed to cope with an environment full of parasitic threats, and when these threats are not present, the immune system turns to attack inappropriate targets.
Recent results from several countries have indicated that children born to farming families are less atopic and suffer fewer allergies than children born to city dwellers. A few studies have tried to quantify this effect; they appear to show that a child will be less likely to suffer allergies the more time he/she spends in the cow barn during the first year of life. This raises an interesting question; is the protective effect due to exposure to specific bacteria associated with domestic livestock or is it simply exposure to microbes in general? There is still no definitive answer to this question, but it has been hypothesized that certain microbial structural components (e.g. bacterial DNA, cell wall lipopolysaccharides) can tip the balance towards activation of "advantageous" Th1 cell-mediated immunity and away from the "damaging" Th2 mediated effects. There is a complex interplay between the various components of the immune system and thus it is too early to translate these findings into practical advice for families.
There is also the possibility that the microbial population in our immediate environment has changed in recent years with this change being attributable to our use of antibiotics. There is evidence that the normal bacterial flora populating our gut has changed radically in the past fifty years. Attempts have been made to rectify this change; pregnant women and newborn infants (especially those designated as high risk cases for allergies) are now being administered prophylactic doses of lactobacilli. The effectiveness of this strategy still needs to be demonstrated, but as is the case in all diseases, prevention is a lot better than cure.
There is a new wind blowing through the world of allergies and their treatment with recommendations almost being turned on their heads. Previously, atopic individuals were told to avoid exposure to potential allergens at all costs (e.g. no pets allowed in the home). Today, many experts believe that the immune system has to be kept active, allowing it to fight against suitable "sparring partners". These theories are still in their infancy, it will be years before we can give practical advice on this matter.
Avoidance of allergens is crucial when an allergy has developed. Thus, half-baked, do-it-yourself techniques intended to shunt the immune system down one pathway may be playing with fire. Furthermore, the hygiene hypothesis should not be misconstrued to believe that it is best to deliberately expose our children to the microbes causing the so-called normal array of childhood diseases by prohibiting their vaccination.
The reason for the huge increase in the incidence in allergy which has occurred in recent years remains something of a mystery. It seems that many more years of research will be needed before we can identify practical and effective solutions to combat these debilitating disorders.
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