NEW YORK — Allergies are mysterious things, especially considering they affect more than 50 million people in the United States. We have a basic understanding of how allergies work — sufferers produce an antibody called Immunoglobulin E when exposed to substances that are otherwise harmless, such as cat dander, peanuts or ragweed. IgE sets off a chain reaction that results in sneezing, sniffling, and red, itchy eyes.
One of the biggest mysteries is why the disease comes and goes, and then comes and goes again. People tend to experience intense allergies between the ages of 5 and 16, then get a couple of decades off before the symptoms return in the 30s, only to diminish around retirement age.
Three types of explanations have been proposed: environmental, infectious and psychological. But as Mitchell Grayson, an associate professor of allergy and immunology at the Medical College of Wisconsin, points out, they're pretty speculative: "They're not quite hypotheses — there's not even enough evidence to call them that."
Most people live relatively stable lives between birth and 18 years of age. Whatever substances they develop allergic reactions to in their early years are likely to remain in their environment as long as they stay in their parents' home. Once they establish independent lives, though, they may get free of what was ailing them for so many years. College dormitories, while cesspools of infection, are relatively hypoallergenic compared to most homes. Tile floors, cheap, plastic-covered mattresses and the absence of dogs and cats may all contribute to the reduction in allergic symptoms.
A more technical explanation for the disappearance of allergies in the late teens involves viruses. When you infect a mouse with certain viruses, its immune system becomes extremely responsive to IgE. Although our immune systems don't work quite like the immune system of a mouse, there are some tantalizing hints that something similar could be at play in the human body. Infants that have been diagnosed with respiratory syncytial virus or even simple cold viruses go on to develop allergies at a significantly higher rate than kids who haven't suffered from these infections. This may also partially explain why most children develop pet allergies at a younger age than they do pollen allergies. Pets are present during cold season, and their dander could cause an immediate immune response while the virus is in the system. Pollen is less likely to be around at the same time as a virus.
Although there isn't much empirical data, some doctors think that the prevalence of allergies and viral infections rise and fall in lockstep as people age. Kids get lots of colds and suffer from allergies. Parents get lots of colds and allergic episodes, mostly because they're surrounded by disease-ridden children. Coincidence? Maybe, maybe not. (Among the many unanswered questions about allergies is whether parents are more likely than non-parents to have allergies.)
A third explanation for the age-related changes in allergies is psychological. Grayson and many other immunologists put the most stock in this theory.
"People in their teens and 20s are interested in chasing other people," says Grayson. "When you're at that invincible age and out having a good time, you're just not bothered by allergies."
People don't like being told that their symptoms are influenced by psychology in any way. But it doesn't mean they're crazy or faking it. Allergies are real, but how a person experiences the symptoms depends on state of mind. The placebo effect is strong in allergy studies. Almost one-third of allergy sufferers report a remission in symptoms when given a sugar pill. For a young adult, there are far more effective placebos in life than sugar pills. There are sports, video games and romance.
Any one of these ideas — environmental, infectious or psychological — could explain the undulating experience of allergies through life. It could also be a combination of these theories, or something else entirely. This is a hot area of research.
Only one thing seems relatively certain: The diminution of allergy symptoms as people head into their golden years has to do with the body slowing down. The immune system is less active in older people, so their IgE response to allergens becomes less pronounced. Other people with weakened immune symptoms, such as women late in pregnancy and people on medications that suppress immunity, also may experience a reprieve from allergy symptoms.
If you're in your 30s and have a history of allergies, you're probably hoping for advice. Unfortunately, there's not much to go on.
Eating dirt won't help you. Several years ago, German immunologist Erika von Mutius noticed that allergies were rare among Bavarian farmers. People who live in close proximity to dirt and manure are exposed to a greater number and diversity of microbes, and so a theory emerged that some combination of bacteria in the gut may protect people against allergies. There is a fair amount of correlational evidence to support the so-called hygiene hypothesis. Allergy prevalence is lower in the developing world than in the developed world and lower in rural than urban environments globally.
Since the hygiene hypothesis became a topic of media discussion, some parents decided to send their kids out to frolic in the dirt in hopes that they won't develop hay fever later in life. But it's called the hygiene hypothesis for a reason — there still isn't a lot of basic science evidence to support it.