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What is an Allergic Reaction?

Most people think of allergies as sneezing, a runny nose or watery eyes.  These are common symptoms of allergy, but allergic disease is actually the product of several events occurring deep within your immune system.

The immune system


The immune system is the body's defense mechanism against pathogens (foreign substances we come into contact with through breathing, eating, and the things we touch).  Some of these pathogens include things like viruses, bacteria and others include things called “allergens.” An allergen is any substance that can trigger an allergic response. Common allergens include plant pollen, mold, animal dander and house dust.  An allergen triggers an allergy because of its effect on certain kinds of antibodies that circulate in your blood stream and other bodily fluids.

Antibodies help to capture unwanted invaders. When allergens first enter the body of a person predisposed to allergies, a series of reactions occurs that trigger the increased production of a specific antibody called IgE antibody so that more and more allergen-specific IgE antibodies are produced.  The IgE antibodies attach themselves to the surface another immune cell called a Mast cells.  When a Mast cell with an IgE attached comes into contact with a substance that is an allergen, the mast cell releases a substance called a histamine, which is irritating to the surrounding tissues.  Mast cells like to live in the nose, the eyes and the lungs.  This is why most people with allergies experience the symptoms that the do: itchy watery eyes, runny noses and asthma (inflammation in their lungs).

Each type of IgE antibody is unique to one specific allergen.  So someone who is allergic to many different things, has many different types of IgE, not just one.  For example, if someone is allergic to milk and also to cat dander, they have at least two different types of IgE, one specific to cats and another specific to milk.

The first time someone who has allergies encounters an allergen, for example someone who is allergic to cats, they don’t have symptoms.  But the immune response is triggered, the IgE is ‘captured.’  This sensitizes the person to that allergen, so for example, the next time that same person comes into contact with a cat, the IgE, now bound to the mast cells, will release chemical mediators such as histamine from that mast cell.  Now the person will experience the symptoms of allergies, such as swelling, itching, sneezing, coughing and asthmatic reactions.

The allergic response or allergic reaction is a cascade.  In other words, it acts like a domino reaction.  Once the mediators are released, causing symptoms and irritation, even more IgE cells bind to mast cells and even more mast cells release histamines… this causes increasing severity of inflammation and allergic symptoms.  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. And it is not known why some people experience allergic diseases and have allergic reactions after exposure to an allergen whereas other people experience only mild symptoms of allergy or no symptoms at all. A family history of allergies is the strongest indicator of whether or not a person will experience allergy symptoms or develop allergic diseases such as rhinitis or asthma.  If one parent has a history of allergic disease or asthma, the estimated risk of the child to develop allergies is 48%; If both parents have a history of allergies or asthma, that risk goes up to as much as 70%.

Types of allergic disease


The existence of IgE antibodies is common to all forms of allergies, but the physical symptoms differ depending where mediators are released. When a person with allergies begins to experience symptoms, some good questions to ask are in order to try to identify the allergen include:  When am I experiencing allergy symptoms.  What allergens am I being exposed to.  How often am I exposed to the allergens causing my allergic symptoms.  Can I avoid these allergens.   

The Most common allergic diseases include the following:

  • 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
  • Problems commonly resulting from allergic rhinitis—sinusitis and otitis media (ear infections)

Diagnosing and treating allergic reactions

An allergist is best qualified to treat allergic diseases. To determine if you have an allergy, your allergist will take a thorough medical history and perform an exam. Following the history and physical exam, your doctor may choose to do allergy testing in order to help verify what allergens are causing your symptoms and to what degree you are allergic to which allergen. 

Copyright © 2008-2010 Dr. Beth Cowan