Types of Allergens Basophils Eosinophils Immunoglobulins Mast cells Mediators Sensitization

Page Topics The Action of a Reaction Who's at Risk? Costs How Allergies Develop Risk Factors Are Emotions to Blame?

Rarely is a medical term used as frequently and casually as "allergy." It pops up in everyday conversation to describe what we don't like – school, work or just about anything else. But medically speaking, allergy has a very specific meaning: a reaction to something that your body doesn't like.

Allergies are an abnormal sensitivity to a substance called an allergen that is eaten, inhaled or touched that most other people can tolerate without trouble. Usually, the immune system does a good job of distinguishing between the many nontoxic substances in our environment and the viruses, bacteria and other troublemakers that threaten health. For example, when harmful bacteria enter the respiratory system, the immune system mounts a defense in which protective proteins called antibodies attach themselves to the outer surface of the bacteria and target them for destruction. Yet it usually leaves alone those substances that don't harm us.

But in people with allergies, the immune system sometimes makes a mistake and launches attacks against a perfectly harmless substance, such as ragweed pollen, animal dander or certain foods or drugs. Hence the word allergy comes from the Greek allos, meaning other. It was first used in 1906 to refer to an "altered reaction" in the body's immune system. Since then, allergies have been called a lot of things ... sometimes in language that would make a sailor blush.

While many mild allergies are more of a nuisance than anything else, they can be disruptive enough to require ongoing vigilance and medical treatment. Occasionally, they can be life-threatening.

The Action of a Reaction

Most people know their allergies by the reaction they cause – the sneezing, wheezing and other effects that are none too pleasing. What they may not know is that what happens to their body is a result of what's happening inside it.

Basically, an allergic reaction is what happens when your immune system tries to do its job: mount a defense against what it believes is a hostile invader. During a period of sensitization, the time when you are exposed to a specific allergen, the immune system produces antibodies – chemicals that are produced by a type of white blood cell whose job is to fight infection and other invaders of the body. Unfortunately, in those with allergies, these cells may produce antibodies against a certain food or pollen.

The body produces five different classes of antibodies, or immunoglobulins – IgA, IgM, IgG, IgD, and IgE – and each class serves a different function. Those formed in response to allergens belong to the IgE class, which normally function to attack parasitic worms. The IgE antibodies bind to the allergen, such as pollen molecules, as well as two types of defensive cells: mast cells (found in the nose, skin, lungs and gastrointestinal tract) and basophils (found mostly in the blood).

These cells then release a series of chemicals called mediators, which cause the sneezing, runny nose, itching and other symptoms of an allergic reaction. Probably the best known of these mediators is histamine.

Because some mediators work very quickly, reactions occur almost instantly after exposure to an allergen; and most reactions occur where the allergens enter the body. So if you inhale an allergen, you'll experience symptoms along the respiratory tract. If you're allergic to a food, you'll likely feel it in your gut, while touching an allergen tends to result in a skin rash.

Some other substances are called in for reinforcement. A type of white blood cell called eosinophils is attracted to the site of the reaction. The result is a more intense reaction – such as swelling or inflammation, which can be more difficult to treat. (Once an allergen enters the bloodstream, though, its effects can be more far-reaching. So a food allergen may not only cause gastrointestinal problems, but can also cause hives or a skin rash.)

Who's at Risk?
If you have allergies, you're in good company. The National Institute of Allergy and Infectious Diseases estimates that 50 million Americans – 1 in 5 of us – has some form of regular allergy.

• Nearly half of people with allergies – at least 10 percent of the entire U.S. population – have hay fever, medically known as allergic rhinitis. This reaction to outdoor airborne allergens like pollens occurs on a seasonal basis. A second type of airborne allergy called perennial allergic rhinitis, occurs year-round and is more of a response to indoor allergens such as pet dander or dust mites.

Costs
All told, allergies rank sixth in cost on the list of chronic diseases in the United States. Each year, Americans lose 3½ million work days because of allergies, at a cost of $639 million. Children lose about 2 million school days. Allergies cost about $2 billion a year in treatment, tests, medications and allergy shots, and require nearly 8 ½ million physician visits.

How Allergies Develop
How people become allergic is still something of a mystery. Experts know it begins with what is called sensitization – a period that ranges from a few weeks to several decades, in which repeated exposure to a particular allergen activates the immune system to attempt to fight what is usually a perfectly innocuous substance. Because this period of sensitization varies so much, one person can develop a contact allergy during infancy while another may not be sensitized until adulthood. Still, most allergies become apparent in childhood – especially inhalant reactions like hay fever. Some children can outgrow one allergy only to develop another later in life. Adults seldom develop new allergies after age 40, but rarely "outgrow" those they have, although sometimes reactions become less severe with age.

Risk Factors
Regardless of when they begin or what symptoms they exhibit, most allergies seem to be caused by the same risk factors:

Heredity
If you have allergies, chances are you're carrying on a family tradition. A child with one allergic parent has a 30 to 50 percent chance of developing allergies, although he may get a different type of allergy than his folks. His chances of developing allergies rise as high as 60 to 80 percent if both parents have them.

Still, children from the same family may not get the same allergies. Studies show that only 25 to 50 percent of identical twins share the same type of allergy. In many cases, one twin will have allergies and the other won't. This means environment is an important factor, too.

Environment
While your genes make you more vulnerable to an allergy, it's your environment that sets it all in motion. Specifically, it's being in a place where you are exposed to high levels of a particular allergen – especially early in life. Infants and young children exposed to a lot of pollen, for instance, are more likely to get hay fever than those less exposed ... even when heredity is taken into account. So if you have allergies and one of your children is exposed to dust mites during infancy, chances are he'll be more likely to develop dust mite allergies than another child of yours who wasn't around mites – or an exposed child of someone not allergic. It's always a combination of both factors that paves the way for allergies to develop.

Are Emotions to Blame?
One of the frustrating aspects of dealing with allergies is the pervasive myth that the disorder is psychosomatic, an "all in your head" phenomenon instead of a "real" medical problem.

Although an allergy is a physical disorder, sometimes reactions can be set off by an emotional or psychological response. For example, studies indicate that people with allergies can develop reactions if they merely revisit a place where they once had a bad allergy attack. This phenomenon, known as a "conditioned response," is not unique to allergies. Patients receiving chemotherapy for cancer may become nauseated just walking into the hospital even if they're not going to see the doctor. As far as allergy is concerned, a conditioned response is as "real" as it would be if the allergen actually were present. Your immune system is simply responding to a signal from your brain triggered by the remembered danger.

Certain drugs – especially those taken for asthma – can cause severe systemic effects as well as emotional or mood changes. Many antihistamines used to control allergy symptoms cause drowsiness (although newer, non-sedating types control allergy symptoms without causing drowsiness). Others can make you restless, nervous or anxious. And the oral corticosteroids used to treat asthma can lead to depression or agitation in some patients.

What's more, managing an allergy can place certain limitations on your lifestyle. Teenagers and young people, in particular, may chafe at restrictions that embarrass them, make them feel different from their friends or less free to enjoy themselves. But even adults can feel uncomfortable if they have to limit activities in order to avoid allergens or irritants (such as cigarette smoke) that can trigger reactions. These restrictions are likely to be more of a burden for people with asthma.

Last updated March 18, 1998

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