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Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School


What Is Allergy?


July 25, 2014

Allergy
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Learn The Basics
What Is Allergy?
What Is Allergy?
htmIsAllergyCore
Allergy is not a disease in the typical sense. It is an exaggeration of the body's natural immune defenses.
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InteliHealth
2011-12-16
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InteliHealth/Harvard Medical Content
2013-12-16

Reviewed by the Faculty of Harvard Medical School

Allergy is not a disease in the typical sense. It is an exaggeration of the body's natural immune defenses. Mild allergies can be annoying, and severe forms can be very serious or even deadly.
 
Allergies vary in the types of symptoms they cause and the best methods to prevent and treat them. The type of allergy you have depends on the allergen that provokes your symptoms (such as mold or cat dander), the specific areas of the body affected by allergy symptoms (such as airways, skin or nose) and whether or not the reaction stays in one place or is triggered throughout the body. For example, allergic asthma, a type of allergy that may be triggered by a variety of airborne substances, is an exaggerated immune reaction confined to the airways of the lung. The symptoms of hay fever or seasonal allergies, caused by pollen in the air, are usually limited to the nose, eyes and sometimes the sinuses or the airways. Similar symptoms can be caused year-round (called “perennial” allergies) by indoor airborne allergens, such as dust mite or cockroach allergens, mold, or pet dander. On the other hand, an allergic reaction to an insect sting can trigger a severe response that affects the whole body. This response, called anaphylaxis, is potentially fatal.
 
Once you learn what type of allergy you have, you can learn to prevent and treat it on your own. You can also learn how to recognize dangerous types of allergic reactions and when to get help.

Three General Rules of Allergy
 
Although each type of allergy behaves differently, some general rules apply:
  • Allergies always involve the immune system.
  • A reaction occurs after exposure to an allergen. The timing may be immediate — soon after contact with an allergen — or delayed — as long as two weeks or longer after a first contact (such as with poison ivy).
  • Prevention is just as important as treatment, if not more.
How An Allergy Occurs
Allergies are immune reactions to normally harmless substances. Think of the immune system as an army. IgE (immunoglobulin E) molecules, antibodies made by the immune system, serve as the soldiers.

These antibodies are on surveillance for the enemy. Normally, IgE antibodies scout for proteins found on the surfaces of worms, fungi or parasite larvae; these proteins are called antigens. When an enemy is engaged, antibodies help mount an attack by activating mast cells, which cause a local inflammatory reaction. Inflammation is an accumulation of cells, fluid and blood that normally helps fight infection or heal a wound, but it may also cause discomfort or swelling.

An allergen is the substance that sets off the immune system in allergy. Allergens are otherwise harmless substances to people without an allergy (dust mites, cat dander, peanuts). People with allergies have high levels of IgE that recognize such allergens as if they were harmful, and stimulate an exaggerated immune response. An allergen "turns on" IgE, which may attach to a mast cell or a basophil. Either of these cells then becomes activated and releases histamines. Histamines cause many of the well-recognized symptoms of allergies, such as itchy skin, itchy eyes and runny nose.

The Role Of the Immune System
 
All allergies are an immune reaction to a normally harmless antigen. An interesting quality of the immune system is that it is capable of learning. Each time you are exposed to an allergen, the immune system takes less time to react. Sometimes, this reaction is more dramatic than the first; in some instances, serious, potentially life-threatening symptoms develop on subsequent exposure to an allergen.
 
Certain allergens are more likely to cause life-threatening symptoms (anaphylaxis) than others. It is important to avoid a second exposure if you have had a reaction in the past to insect stings, peanuts, latex or penicillin.
 
The role of the immune system in an allergy defines how that allergy is treated. All allergy treatments act by blocking immune system signals, such as histamines, or calming the immune system reaction (with corticosteroids).
 
The Timing Of The Allergic Reaction
 
Most allergic reactions occur shortly after you have been exposed to an allergen. For example, seasonal allergies are likely to worsen on the same day that the pollen count goes up. Or, if you have a cat allergy and visit the home of someone who owns a cat, you're likely to start developing an allergic reaction before you leave the house.
 
However, some allergic reactions take longer to occur. For example, some forms of contact dermatitis (such as poison ivy allergy) may take longer than a week to show symptoms if you have had a first exposure. Usually the reaction is much more immediate if it is not the first time you have been exposed and had a reaction.
Prevention
 
Prevention is your best defense. Avoiding an allergen or treating your symptoms early is easier than trying to calm a reaction that has progressed out of control.

Common Misconceptions About Allergies
 
"All reactions to medications or foods are allergies." There is a big difference between having an allergy to something versus an intolerance or side effects. People often use the term "allergy" loosely, referring to any reaction to medication or food as an allergy, but it is a mistake to use the word “allergy” for many of these reactions. Medication and food reactions can also be side effects or an intolerance (such as stomach or intestinal upset) rather than an allergy.
 
True allergies involve the immune system. Because immune-system reactions can be dangerous or life threatening, it is extremely important to know if you have an allergy or an intolerance or side effect. Allergies to antibiotics such as penicillin or sulfa are often diagnosed by the appearance of a rash after taking the medication. If you take the medication a second time, the immune-system reaction can be much more severe, even life threatening. On the other hand, an antibiotic such as erythromycin is commonly associated with stomach upset or diarrhea that is not related to the immune system. These symptoms are a side effect of this medication, not an allergic reaction. Because most symptoms from erythromycin are side effects, it is usually OK to take this medication or other related medications in the future. Always tell your doctor or any other health-care provider about any reactions to medications every time you are prescribed a medication.
 
If you have ever had a reaction to a medication or food, make sure you clarify with your health-care provider whether you have an allergy or an intolerance. It will make a difference how carefully you avoid something and whether you need to take special precautions in case of an emergency.
 
"I am allergic to vaccines." Another common misconception is that vaccines can cause allergic reactions. The site of a vaccine injection can appear red, itchy or painful, but this is usually the result of skin or muscle irritation or an infection at the injection site. The exceptions to this rule are vaccines that contain egg proteins. These vaccines can cause allergic reactions and should not be given to people with egg allergies.
 
The key to identifying true allergies is to give your health-care provider a precise description of your symptoms, including the amount of time between the reaction and the exposure to a specific medication, food or other medical treatment.
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