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Allergies are not only becoming more prevalent but, in the case of food allergy, the natural course may be changing, according to Robert A. Wood, MD, professor of pediatrics and director of pediatric allergy and immunology at the Johns Hopkins University School of Medicine in Baltimore.
Among allergic diseases, food allergies have the quickest onset, with peak prevalence at 1 year of age. In contrast, asthma and allergic rhinitis have a much more gradual onset, and do not hit peak prevalence until adolescence or early adulthood. A child who develops a food allergy or AD early in life is much more likely to go on to develop other allergic disease, including asthma and allergic rhinitis, as they grow older.
"This natural progression of the allergy-prone child from atopic dermatitis and/or food allergy to asthma and allergic rhinitis is referred to as the 'atopic march'," said Dr. Wood. "This is a very real phenomenon."
Studies show that the prevalence of asthma and AD rose dramatically in the 1980s and 1990s but stabilized around by the end of the 1990s. Now, there is evidence that the prevalence of peanut allergy has doubled in the last five to 10 years; the data are less clear on other foods, but Wood recently completed a study in which milk allergy appears to be undergoing changes.
"We used to tell people that there was a 90% chance that kids will outgrow milk allergy; our new study shows that 70% of milk allergies now persist beyond the age of 12 years," Dr. Wood explained. "That indicates that what we are seeing now is different than from what studies found 20 or 30 years ago, and an indication that the natural course of food allergy may be changing."
Wood said that because the changes in allergy prevalence have been so dramatic, genetic factors are probably not involved. He reviewed the so-called hygiene hypothesis, which proposes that the T-helper-2 cells, which predominate in a newborn, develop over the first six months of life in one of two directions, and become predominantly T-helper-1 (no allergy) or remain predominantly T-helper 2 (allergy develops). The immune system of children who have an older sibling and are exposed to a variety of infections do not tend to develop allergies. Children who do not have a sibling and are not exposed to many few infections do tend to develop allergies.
Supporting the hygiene hypothesis is evidence that children who are raised on a farm, attend day care, have at least two older siblings, live in a less-developed country, and are exposed to parasites or pets, have an immune system that resists the development of allergy. Dr. Wood points out, however, that inner-city children defy this profile: They have all the "benefits," as it were, of poor hygiene, such as older siblings and exposure to factory by-products, yet the prevalence of allergy among them is on the rise. What exactly is overwhelming the potential benefits of bad hygiene for these children? That, Dr. Wood said, is a question immunology researchers are trying to answer.
The answer may be specific to Western countries, as evidence suggests that allergy rises rapidly as countries become "westernized." Woods gave the example of East and West Germany after the Berlin wall came down in the 1990's. Before that, allergies were one quarter as common in East Germany as they were in West Germany. East Germany was much more polluted, yet had much less allergy. Within 10 years of the Berlin Wall coming down, prevalence in the East and in the West equalized.
Other variables affecting the development of allergy are allergen exposure, housing variables, exposure to microbial agents, lifestyle factors, exposure to tobacco smoke, and dietary variables.
"We generally think of air pollution as being bad for the immune system, but that may not necessarily be the case," Dr. Wood explained. "For air pollutants other than endotoxin and tobacco, it is much harder to determine a role in allergy development, especially when you see examples like the East and West Germany scenario, in which it was clearly much worse in the country where there was much less allergy."