Health

What is an allergy?

An allergy is, in a sense, an overreaction of the immune system. When a person has an allergy, the immune system produces antibodies in response to a usually benign substance.

“Normally the body gets accustomed to allergens,” says Rhoda Kagan, a paediatric allergist at The Montreal Children’s Hospital. “Many of us make antibodies to food, for example, but we don’t become allergic because we make non-detrimental antibodies.” The allergy-related antibodies, on the other hand (known as immunoglobulin E or IgE), become bonded to mast cells — found in the lungs, nose, skin and intestines — which release chemicals called mediators. These mediators — the best known of which is histamine — lead to symptoms such as hives, rashes, wheezing, runny nose, itchy eyes, vomiting and diarrhea (in young children). In extreme cases, this can cause anaphylaxis, also known as systemic or generalized allergic reaction — a life-threatening reaction characterized by difficulty breathing and itching or hives.

At what age do allergies usually appear?

Allergies can appear any time in life, although they usually show up during childhood. Often the first sign of allergic tendencies is chronic eczema (atopic dermatitis). Babies with this condition often go on to develop allergies, asthma or both. This is part of what is sometimes called the “allergic march”: Eczema and food allergies in infancy and toddlerhood often evolve into asthma and allergic rhinitis (hay fever, dust mite allergy) later in childhood. Not every kid develops all of these conditions, but many asthmatic children also have allergies.

Food allergies tend to appear in the first few years of life, while environmental allergies (dust, pets, pollen, moulds) generally don’t arise until school age. “It’s unusual to see seasonal allergies in children under five,” Kagan says.

Do allergies ever go away?

Food allergies can, but environmental allergies tend not to. Milk, egg, soy and wheat allergies are the most commonly outgrown. “Eighty-five percent of children outgrow milk allergy by age three or four,” Kagan says. “Even peanut allergies can be outgrown.”

In a small study at BC Children’s Hospital, 15 kids who had been identified with peanut allergy as infants were “challenged” — that is, given small amounts of peanut butter under close medical supervision. Four showed no reaction. “This study has limitations because of its small size and the fact that we only went up to five grams [about one teaspoon] of peanut butter,” says the study’s co-author, Alexander Ferguson, head of the hospital’s allergy division. “But it does support other work that suggests up to 20 percent of children will outgrow peanut allergy.”

Another condition that often improves is eczema. Although these children will usually develop other allergies, 95 percent of the eczema itself clears up by puberty.

What’s the difference between a food allergy and a food intolerance?

An allergy is a response of the immune system, while an intolerance is not. A food intolerance can cause cramping and nausea as well as symptoms that are similar to allergic reactions, but these have nothing to do with the immune system. Lactose intolerance, for example, is caused by a deficiency of the enzyme that breaks down the sugar in dairy products.

The difference is significant because food allergies can cause anaphylaxis, but intolerances cannot. In other words, although a food intolerance may at times make you wish you were dead, it won’t actually kill you. Food allergies, in rare circumstances, can be life-threatening. Furthermore, an allergy usually requires you to completely avoid a food, whereas a child with an intolerance may be able to handle small quantities. Children also tend to outgrow food intolerances more quickly.

What causes the scary allergies, like peanut and bee sting?

“This is the million-dollar question,” says Jane Salter, president of Anaphylaxis Canada. “I don’t believe that it’s random, as is often suggested. It’s just that we don’t know all the reasons yet, so we can’t predict who will get a peanut allergy and who won’t.”

Genetics clearly play a role. Having a parent with the allergy doubles the risk, and having a sibling with the allergy makes a child six to ten percent more likely to develop a peanut allergy. There is also a genetic component to insect sting allergy, though it’s not as strong.

How many children have peanut allergy, and how many die from it?

Salter estimates that peanut allergy affects one child out of every 100 to 150. Not all of these kids will have had a life-threatening reaction. “However,” Kagan explains, “since about half of children whose first reaction is mild will go on to have a severe reaction later, you can’t really say that there’s any such thing as a mild peanut allergy.”

Children seldom die from peanut anaphylaxis, however. In fact, the group that seems to be most at risk is teenagers and young adults. Ontario coroners’ records show 63 deaths from anaphylaxis between 1986 and 2000, 32 of which were food-related. Eleven victims (ten peanut, one sesame seed) were children under 18, but none were under nine. Six died from anaphylaxis in a school or camp setting, but there has not been a single death in either of those settings since 1994. Salter believes this is due to increased awareness and better management of anaphylaxis by parents and caregivers.

Can allergies cause ear infections?

Indirectly. Allergies can help create conditions that make ear infections possible — specifically, they contribute to the presence of ear fluid.

A team of Canadian researchers, led by paediatric otolaryngologist Steven Sobol at McGill University, did a study involving 26 children with chronic fluid in the middle ear. Eight had allergies (none were food allergies). The team found that the ear fluid of these eight children contained cytokines (molecules that cause disease) associated with allergy, while the fluid of the other 18 contained cytokines that are not. “This is pretty direct evidence that allergies can create the conditions that cause middle-ear disease,” says Sobol. “The allergy creates inflammation in the middle ear, which causes the excess fluid.” Excess ear fluid, in turn, can cause hearing loss or recurrent infections.

What is the best way to detect allergies?

Most allergies are first detected when a parent observes a reaction to a food, or the child seems to have ongoing colds or runny noses during pollen season. The first-line tool for confirming the allergy is still the old skin prick test: The allergist exposes a tiny area of scratched skin to an allergen, and a positive result shows up in a local reaction resembling a mosquito bite. The test is easy, inexpensive and quick — you have an answer within 15 minutes. If there’s no reaction, there’s no allergy 95 percent of the time. The drawback is that there are a lot of false positives — up to 50 percent. For example, a child can outgrow a food allergy but still test positive to a skin prick for years.

For food allergies, the ultimate arbiter is the “challenge,” where patients are given small amounts of the suspect food under medical supervision and carefully controlled conditions. If the challenge doesn’t produce a reaction, there is no allergy, no matter what the skin prick test says.

Challenges can be risky, however, so allergists sometimes use blood tests. There are two kinds: IgE RAST and Immunocap. (Both measure the amount of IgE in the blood; the higher the amount, the greater the likelihood of allergy. RAST is an older technology and Immunocap is considered more accurate.) One problem with blood testing is that it’s expensive and not usually covered by provincial health insurance. Another is that, while the test can confirm the likely presence of an allergy, it cannot predict the severity of reactions. Still, the tests are useful for confirming suspected cases of severe food allergy, and they can help determine if a child has outgrown an allergy.

Blood tests may also indicate whether it’s safe to challenge a child: A low IgE level means it might be safe to try, although there is still some debate about this. Monika Gibson, coordinator of the Ontario chapter of the Allergy/Asthma Information Association, summarizes recent research on peanut allergies: “One US study suggested that [an IgE level] below 0.3 meant it was safe to try a challenge. However, some physicians were getting into some pretty hairy situations using those values. A Dutch study suggested that 0.12 was a safer benchmark, but I think they’re still trying to figure it out.”

Can you do anything to prevent, or lessen the likelihood, of allergies?

Maybe, but not with any certainty.

To develop an allergy, a child must first become “sensitized” to the allergen — that is, exposed to it so the body manufactures the IgE antibodies that cause future reactions. This is why mothers in families with a history of allergy are often advised to avoid certain foods during pregnancy and while breastfeeding, and to delay the introduction of these foods in the baby’s diet. The question is, how well does this strategy work?

Kagan is cautious. “There is a lot of consensus that it’s a good idea, but little evidence that it works.” For one thing, doctors can predict, based on family history, which children are most likely to develop allergies, but not the specific types they will get. If a parent has hay fever, for example, the child may end up with that same allergy, or asthma, eczema, food allergies or none at all. So it’s hard to know if delaying foods or other prevention tactics will make a difference.

Another problem, according to Toronto allergist Milton Gold, is that people have taken an idea that works fairly well with respect to milk allergy and generalized it. “Fifty percent of babies who have a milk allergy will outgrow it by age one. So if you avoid cow’s milk [including cow’s-milk-based formula] until age one, half the kids who would have developed a milk allergy would have already outgrown it. You didn’t really prevent the allergy, you avoided it.” However, this doesn’t work as well for other allergenic foods, and if you restrict a number of foods — milk, eggs, wheat, soy, peanuts, tree nuts (all nuts other than peanuts) and fish — you severely limit your child’s diet. So any potential allergy reduction must be balanced against the child’s nutritional needs. Recent research indicates that many children with food allergies have nutritionally inadequate diets.

If there’s a severe food allergy in the family, it makes sense to avoid that food during pregnancy, breastfeeding and early in the child’s life. Babies with a family history should not start solids until six months (recommended for all babies, but especially important when allergies are a factor). Introduce new foods one at a time, not to prevent allergies but to identify them. But such strategies aren’t necessary for most families. “People need to be clear that you don’t cause an allergy by giving a child a food,” says Gold. “The predisposed child will likely be exposed to the allergen one way or another.” He says if the parents are worried, it’s more prudent to introduce food in a controlled fashion, or have the child tested. “Somehow it’s crept out there that at three years children are magically protected [from peanut allergy], and that’s absolutely not true. A child can still be sensitized later in life.” There is some evidence that reducing exposure to dust (dust mite feces are the actual culprit) in the first year can mitigate that allergy. Families with an allergic history might consider whether baby’s room really needs that plush carpet, piles of fluffy bedding and dozens of stuffed toys. Limiting a fetus’s or infant’s exposure to the effects of tobacco offers some protection against allergies as well.

Finally, what about breastfeeding? “I’ve had women in my office who say, ‘I breastfed. How can my baby have allergies?’” Gold says. Although it is controversial in scientific circles, the weight of evidence does suggest that breastfeeding can delay the onset of eczema and asthma for some children. But like any other preventive measure, it’s not a sure thing. “What you may be able to achieve is a modification but not complete protection.”

Why do so many more people have allergies than in the past?

There are several theories about this, but nothing has yet been proven.

Some believe that air pollution is responsible for the rise in asthma (which increased by 75 percent between 1980 and 1994, according to the US Centers for Disease Control). Others say indoor air is the problem: In our more tightly sealed, carpet-lined and pet-laden houses, we trap potential airborne allergens inside, where children are sensitized to them.

The theory that’s received the most attention is the so-called hygiene hypothesis. It says that in our sanitized world, with germ-killing cleaners and antibiotics, our immune systems do not have enough natural enemies to fight and therefore are more likely to overreact to benign substances. Studies have shown, for example, that kids who are exposed to more germs — farm children, those who attend daycare, youngest siblings in large families and, in general, children in less-developed countries — actually have lower rates of allergies and asthma.

Want More Info?

• Allergy/Asthma Information Association, 1-800-611-7011; www.aaia.ca.
• Anaphylaxis Canada, 1-800-785-5666;www.anaphylaxis.org .
• Asthma Society of Canada, 1-800-787-3880; www.asthma.ca .

Originally published in Today's Parent, October 2002