What is an allergic reaction?  Back to top↑
Allergy (also known as immediate hypersensitivity) is defined as an "abnormal sensitivity to a substance which is normally tolerated and generally considered harmless." While all immune responses occur as a result of exposure to foreign substances, allergic reactions are distinct from the protective or enhanced "immunity" conferred by immunizations or natural infection. It is estimated that about 50 million American suffer from allergic disease of one form or another, and the incidence of these illnesses is increasing.

The immune system serves as the body's defense mechanism against the countless different foreign ("non-self") substances present in the air we breathe, the foods we eat, and the things we touch.  Within this immense group of foreign materials, the term "allergen" refers to those substances that primarily result in an allergic immune response.

An essential part of the human immune system is its ability to develop immunologic recognition and memory.  Once the cells of the immune system have encountered a foreign substance and recognized it as "non-self," that initial contact will forever be "remembered."  If that specific substance is ever encountered again, the body's response to it will be much more rapid and intense as a result of chemical mediators produced by the memory cells that become activated upon re-exposure, and which, in turn, further amplify the response by activating other parts of the immune system.

Another mechanism by which the immune system helps to defend us against foreign materials involves the production of millions of different antibodies (also called immunoglobulins.)  Each individual antibody has the ability to recognize and bind to one specific and unique foreign substance.  Antibodies circulate in the bloodstream, and are present in almost all bodily fluids, where they help to "capture" and prevent entrance of unwanted foreign matter.

In humans, it is the IgE class of antibodies which is responsible for the majority of allergic reactions.  Although allergic individuals often have much higher levels of IgE in their blood than do non-allergic individuals, values overlap widely between these two groups.  An allergic individual may have very high levels of IgE antibodies for one or a few specific allergens, without having elevated levels of total IgE in their blood.  Thus, the diagnostic value of using the total serum IgE concentrations as a screening test or allergic diseases is limited.

It is not yet fully understood why some substances are allergenic and others are not, nor why all individuals do not develop an allergic reaction after exposure to allergens.  However, there may be a genetic contribution to allergic diseases, and children whose parents have allergies have an increased likelihood to also develop allergic diseases.

An individual who has developed IgE antibodies that recognize one or more allergens (pollens, molds, animal danders, dust mites, etc.) is referred to as having become sensitized to these allergens.  The allergen-specific IgE molecules travel through the blood and into the tissues, where they coat the surface of mast cells.  As many as 500,000 IgE antibodies with distinct specificities may be present on the surface of a single mast cell, thus enabling each cell to recognize many different unique allergens.

Mast cells, which are particularly abundant in the lining of the nose, eyes, lungs and gastrointestinal tract, become activated when allergen molecules make physical contact with the IgE antibodies on the surface of the cell capable of recognizing those specific allergens.  Thus, the mast cells of an individual who had developed IgE antibodies to cat dander only would not become activated by exposure to ragweed pollen.An allergic reaction starts when allergen molecules come into contact with and activate mast cells coated by the allergen-specific IgE.  Once activated, mast cells release a variety of potent chemical mediators, all of which have power proinflammatory properties.  These include chemicals such as histamine (hence the therapeutic use of antihistamines), leukotrienes and prostaglandins, as well as cytokines (protein molecules which serve as regulators of cellular interactions.)

One of the most important recent advances has been the recognition that allergic reactions produce inflammation of the tissues in which they occur.  An allergic reaction unleashes a cascade of events, beginning with the release of chemical mediators from the activated mast cell.  These then recruit other inflammatory cells from the bloodstream to invade these areas where they, along with nearby resident cells, release additional chemical mediators, and ultimately result in inflammation of the involved tissues.  Much of the symptomatology of chronic allergic diseases, such as swelling, excess mucus gland activity, and hyperresponsiveness to irritating stimuli, is though to result from perpetuation of tissue inflammation due to ongoing exposure to allergens.

The key questions for each allergic individual are: What allergens does my immune system specifically recognize? When (and how much) am I exposed to these substances?  The allergic inflammatory response that results from exposure to these relevant allergens, with the accompanying swelling and sensitivity of the exposed surfaces, is directly responsible for clinical allergy symptoms.

Allergic rhinitis - Commonly referred to as "Hay fever", this occurs as a result of allergens touching the lining of the nose of an individual sensitized to that particular allergen.  The accompanying symptoms, such as nasal congestion, itching and dripping, are due to the localized allergic inflammation of the exposed nasal mucus membranes.  Daily, ongoing exposure perpetuates the inflammatory response and the symptoms.  Similarly, allergic conjunctivitis represents the localized allergic inflammatory response of the eyes, while atopic dermatitis, or eczema, may often result from allergen exposure to the skin.  For more information visit

Asthma - A chronic lung disease characterized by coughing, chest tightness, shortness of breath and wheezing due to a reversible obstruction of airflow resulting from inflammation and hyperresponsiveness of the airways.  In sensitized individuals, inhalation of allergens may produce allergic inflammation of the airway lining, and precipitate a flare-up of asthma.  Asthma may also occur as a result of other inflammatory stimuli, such as respiratory tract infections.  For more information visit

Food allergies - Individuals who have become sensitized to specific foods may have severe and possibly life threatening reactions after ingestion of these substances.  Common triggers include shellfish, eggs, milk, wheat, peanuts, and other nuts.  Food allergy occurs frequently in children, and it is not uncommon for individuals to lose their food hypersensitivity over time.  For more information:

Anaphylactic shock - The most severe of all allergic reactions, this systemic response causes swelling throughout the body, with life-threatening and sometimes fatal consequences due to extreme swelling and constriction of the throat and airways, as well as a sudden drop in blood pressure.  Anaphylaxis often occurs in people particularly sensitive to penicillin, stinging insects, shellfish, peanuts or latex.  Prompt administration of epinephrine (adrenalin) is essential in the initial treatment of anaphylaxis.  For more information visit

Allergy skin testing or RAST blood testing are used to determine precisely which allergenic molecule(s) are recognized by the patient's immune system. Once identified, optimal therapy involves minimizing exposure to these allergens to whatever extent possible, combined with medications to both reduce allergic symptoms as well as the inflammation of the involved tissues. In addition, allergy shots (immunotherapy) may be used to modify the immune response, thus diminishing the intensity of allergic reactions occurring from natural exposure to allergens.