Mahendra P. Dadhania, MD     Shailen R. Shah, MD     Adele M. Guernica, DO

Board Certified by American Board of Allergy & Immunology
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What is an Allergic Reaction

Tips to Remember: 

Approximately 50 million Americans suffer from some form of allergic disease, and the incidence is increasing. When most of us think of allergies, we think of sneezing, a runny nose or watery eyes. While those are symptoms of some types of allergic disease, an allergic reaction is actually a product of several events occurring within your immune system.

The immune system
The immune system serves as the body's defense mechanism against the countless different substances present in the air we breathe, the foods we eat, and the things we touch. Within this immense group of materials, the term "allergen" refers to any substance that can trigger an allergic response. Common allergens include pollen, mold, animal dander, and cockroach and house dust mite droppings.

Antibodies circulate in the bloodstream and are present in almost all bodily fluids. They help to capture unwanted invaders. People who have allergies have an antibody called Immunoglobulin E, or IgE. When allergens first enter the body of a person predisposed to allergies, a series of reactions occurs and allergen-specific IgE antibodies are produced. Following IgE production, these antibodies travel to cells called mast cells, which are particularly plentiful in the nose, eyes, lungs and gastrointestinal tract. The IgE antibodies attach themselves to the surface of the mast cells and wait for their particular allergen.

Each type of IgE has specific "radar" for one type of allergen only. That's why some people are only allergic to cat dander (they only have the IgE antibodies specific to cat dander) and others seem to be allergic to everything (they have many more types of IgE antibodies.)

The next time an allergic individual comes into contact with the allergens to which he or she is sensitive, the allergens will be captured by the IgE. This initiates the release of chemical "mediators" such as histamine and other chemicals from the mast cells. These mediators produce the symptoms of an allergic reaction, such as swelling of tissues, sneezing, wheezing, coughing and other reactions.

The allergic reaction typically continues: these newly released mediators recruit other inflammatory cells to that site, resulting in additional inflammation. Many symptoms of chronic allergic disease - such as swelling, excessive mucus and hyperresponsiveness to irritating stimuli-are the result of tissue inflammation due to ongoing exposure to allergens.

It is not yet fully understood why some substances trigger allergies and others do not, nor why every person does not develop an allergic reaction after exposure to allergens. A family history of allergies is the single most important factor that predisposes a person to develop allergic disease. If one parent has allergic disease, the estimated risk of the child to develop allergies is 48%; the child's risk grows to 70% if both parents have allergies.

Types of allergic disease
The existence of IgE antibodies is common to all forms of allergic disease. But the physical symptoms differ depending on the type of mediator released and where they act. When a person with allergies begins to experience symptoms, key questions are: What allergens am I reacting to? When, and how much, am I exposed to these substances?

Major allergic diseases include:

  • Allergic rhinitis, or "hay fever;"
  • Allergic conjunctivitis (an eye reaction);
  • Asthma;
  • Atopic dermatitis, or allergic skin reactions;
  • Urticaria, also known as hives;
  • Severe allergic reactions to substances such as food, latex, medications, and insect stings;
  • And problems commonly resulting from allergic rhinitis-sinusitis and otitis media (ear infections).

Symptoms of allergic rhinitis, commonly referred to as "hay fever," occur when allergens touch the lining of the nose - called the mucus membranes - of a person who is sensitive to that particular allergen. Allergic rhinitis is characterized by congestion, itching and dripping of the nose and itchy, watery eyes. If the person continues to have daily, ongoing exposure to the responsible allergen(s), the symptoms will continue. Similar to allergic rhinitis, allergic conjunctivitis occurs when the eyes react to allergens with symptoms of reddening, itching and swelling.

Symptoms of atopic dermatitis, also called eczema, often result from allergen exposure to the skin. Dermatitis symptoms include itching, reddening, and flaking or peeling of the skin. Symptoms begin in childhood for 80% of those with atopic dermatitis. Dermatitis often precedes other allergic disorders; over 50% of those with atopic dermatitis also develop asthma.

Urticaria, or hives, another allergic skin reaction, is characterized by itchy red bumps that can occur in clumps and be either large or small. Hives are often triggered by infection, certain foods or medications. Foods commonly associated with hives include nuts, tomatoes, shellfish and berries. Medications include penicillin, sulfa, anti-seizure medications, phenobarbital and aspirin.

Asthma is a chronic lung disease that affects more than 17 million Americans and is characterized by coughing, chest tightness, shortness of breath and wheezing. Allergic rhinitis is considered a risk factor in developing asthma: up to 78% of those with asthma also have allergic rhinitis.

When an individual experiences asthma symptoms, the inflamed airways become constricted so it becomes more difficult to breathe through the narrowed air passages. For those with allergies, inhaling allergens may produce increased inflammation of the airway lining and further narrowing of the air passages. Asthma may also occur as a result of other stimuli, such as respiratory tract infections or exposure to irritants.

Individuals who have allergies to specific foods may have severe and possibly life-threatening reactions if they eat them. The most common food allergy triggers are the proteins in cow's milk, eggs, peanuts, wheat, soy, fish, shellfish and tree nuts. Food allergy occurs more frequently in children than adults. Many of these children may lose their sensitivity to particular foods over time.

Foods and other allergens, including penicillin, insect stings and latex, can trigger a severe, systemic allergic reaction called anaphylaxis. Anaphylaxis is caused by swelling throughout the body, and can involve several organ systems. Symptoms of anaphylaxis include a feeling of warmth, flushing, tingling in the mouth or a red, itchy rash. Other symptoms may include feelings of light-headedness, shortness of breath, severe sneezing, anxiety, stomach or uterine cramps, and/or vomiting and diarrhea. In severe cases, patients may experience a drop in blood pressure that results in a loss of consciousness and shock. Without immediate treatment with an injection of epinephrine (adrenalin), anaphylaxis may be fatal.

Sinusitis and otitis media are other common allergic diseases that often are triggered by allergic rhinitis. Sinusitis is an acute or chronic inflammation of the nasal sinuses, which are hollow cavities within the cheek bones found around the eyes and behind the nose. This condition affects over 15% of the U.S. population. Otitis media - or ear infections - is the most common childhood disease requiring physician care. If not properly treated, it can affect a child's speech and language development. You can learn more about various allergic diseases by reading the appropriate Tips brochures in this series.

Diagnosing and treating allergic reactions
An allergist/immunologist is best qualified to treat allergic diseases. To determine if you have an allergy, your allergist/immunologist will take a thorough medical history and perform an exam. If indicated, he or she will then perform allergy skin testing, or sometimes blood testing, to determine precisely which substance is causing your allergy. Once your allergy triggers are identified, your physician can help you establish a treatment program. The first step is to minimize your exposure to your particular allergens to whatever extent possible. Your physician may also prescribe medications to reduce allergic symptoms as well as inflammation.

Immunotherapy, also known as allergy shots or vaccinations, may also be recommended to modify your immune response. For many patients, immunotherapy can help diminish their reactions to allergens. Please see the Tip brochure in this series for more information on allergy shots.

In summary, an allergic reaction is a complex chain of events that involves many cells, chemicals and tissues throughout the body. While there is not yet a cure for allergic disease, there are many treatments available to lessen symptoms. See your allergist/immunologist to determine the best course of treatment.

Your allergist/immunologist can provide you with more information on allergic reactions.

Tips to Remember are created by the Public Education Committee of the American Academy of Allergy, Asthma and Immunology. This brochure was updated in 2003.

The content of this brochure is for informational purposes only. It is not intended to replace evaluation by a physician. If you have questions or medical concerns, please contact your allergist/immunologist.

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Asthma and Immunology
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