Whether they are food, respiratory, or dermatological, allergies have been on the rise since the 1970s, even more so in the past 10 years. They particularly affect young children, although they can occur at any age. What are the causes? Research has some answers, although much remains to be done.
Allergies: Risk Factors
Until recently, allergists believed that allergies were hereditary and that an allergic predisposition determined at birth could never disappear. Since the early 1970s, the resurgence of allergies contradicted this hypothesis. Indeed, the accelerated development of allergies (allergic rhinitis, asthma, eczema, etc.) could not be solely attributed to heredity. Today, researchers attribute this acceleration to the environment. Western countries are the most affected by a sharp rise in allergies. Therefore, scientists assume that it is the lifestyles and environmental conditions of these countries that are being questioned. Although scientists have not yet reconstructed the entire puzzle, their conclusions are unequivocal.
According to Andrew Weil, director of the Integrated Medicine Research Program and professor of clinical medicine at the University of Arizona, Tucson College of Medicine, "it is not possible to eliminate a hereditary predisposition to allergies, however, it is possible to act on the immune system by choosing an appropriate lifestyle and modifying the environment. Allergy is an immune system response to a supposed invasion of the body, so it is important to act so that the supposed enemy is once again considered benevolent."
Allergies: Genetic Predispositions
Allergy occurs more often in genetically predisposed individuals, meaning that most of the time, one or both parents suffer or have suffered from allergies. This is referred to as an atopic terrain or atopy. The risk of becoming allergic increases depending on family history, with a risk of 75% if both parents are allergic. It is 25 to 40% if only one parent is allergic. When there is no family history, the risk is only 12 to 15%. Within the same family, allergic manifestations can be similar or different (eczema, hives, allergic rhinitis, allergic asthma) but are generally of the same category.
Interleukin-4 is one of the proteins that plays an essential role in allergic reactions. It increases the production of IgE antibodies, which promote the onset of bothersome symptoms (swelling, difficulty breathing, rhinitis, coughing). Some scientists hypothesize that the predominance among African Americans of a gene that promotes the production of interleukin-4 could be the reason why they suffer from some of the highest rates of allergy and asthma in the United States. Indeed, some studies show that African Americans have very high levels of interleukin-4. Some scientists link this predominance to the fact that their ancestors probably needed it to fight tropical parasites. This predominance still exists today as these numerous interleukin-4 proteins now attack harmless allergens.
Allergies: Environment and Lifestyle
Time and time again, researchers have found, among groups of the same racial origin, radically different allergy rates depending on the typology of the environment where they lived. In most cases, there is a resurgence of allergies in wealthy and urbanized environments. The pattern holds true in Asia, where highly urbanized Japanese are more allergic than rural Chinese. It is also true in Africa, where metropolitan residents are more allergic than people living in the bush.
One of the most telling studies was conducted in Germany, just after the reunification of East and West Germany. According to Harold Nelson, physician and researcher at the National Jewish Medical Center in Denver, it was found that Western Germans, richer than their Eastern cousins, suffered much more from allergies than the latter. This observation then swept away the assumption that pollution, to which East Germans were much more exposed than those in the West, bore a heavy responsibility for the development of the allergy epidemic. However, apart from the case of passive indoor smoking, which does indeed increase the risk of asthma in children, the involvement of air pollution in the development of asthma and other forms of allergies is not proven.
On the other hand, there is a strong correlation between allergies and other environmental factors such as socioeconomic status, family size, childhood infections, and diet. If there was any doubt that the environment was not a driving force in allergy, what happened in ex-East Germany in the 10 years following reunification allows to dispel it. The capitalist model became the norm there, the standard of living dramatically increased, and so did the rate of allergies. Today, the whole of Germany is grappling with extremely high rates of allergies.
When scientists identified allergy as a disease of Western life, they thought the explanation was simple: houses are filled with all sorts of things, including indoor pollution, pets, and processed foods. "We first thought that the increasing prevalence of asthma was related to the quality of indoor air in homes (better insulated, overheated or humid, insufficient air renewal) or their location (crowded buildings, terraced houses, etc.)," says Nelson. "There is no doubt that the quality of indoor air in homes has deteriorated with westernization. Everyone thought they had found the overall answer. Then we began to see situations where this hypothesis did not fit." As evidence, Nelson cites European studies showing that infants exposed to dogs or cats at home developed 50% fewer allergies than children from pet-free households. This observation led to the hygiene hypothesis.
The hygiene hypothesis suggests that our overly sanitized Western lifestyle keeps the immune system uncertain, unbalanced, and unable to distinguish friends from foes. Researchers believe that much of this confusion occurs in childhood. It is believed that a baby's immature immune system can only develop properly if exposed to bacteria from fermented foods. Furthermore, premature exposure to antibiotics seems to create the wrong stimulus by disrupting the balance between friendly and hostile bacteria. The result is an increased risk of allergies. The challenge for scientists is to understand how the Western, overly sanitized lifestyle, genetics, and the environment combine to create allergies. But, since two of these three factors are controllable, there is great hope that one day allergic individuals can live (almost) like everyone else.
Sources : Prevention Magazine. (1) La médecine intégrative combine la médecine classique à la thérapie naturelle et traditionnelle (médecine chinoise, tradithérapie Indienne ou Africaine, Ayurveda, phytothérapie, etc.).