When the normal equilibrium of the human body is threatened by an external agent, a sequence of automatic defense mechanisms moves into action. If the body becomes overheated, for example, it begins to perspire in order to cool itself. If disease-causing bacteria or viruses invade, the body's immune system produces protective antibodies. An allergy represents a response by the immune system to agents perceived as possibly dangerous to the body. But in this case, the response to the agent, known as an allergen, is excessive, and the reaction is neither normal nor desirable.

There are literally hundreds of possible causes of allergy, and the reaction may express itself in different ways. One person breaks out in hives after eating strawberries. Another starts to sneeze in the presence of cats. A third develops itchy, watery eyes in reaction to dust. In people who suffer from asthma, exposure to an allergen may trigger sneezing, shortness of breath, wheezing, or even sudden death.


The symptoms of allergy may first appear at any time from infancy to old age. They occur most often before the age of 20. One of the major problems facing doctors who treat allergies is to recognize the condition early enough so that treatment can be started when it can do the most good. Many allergic reactions are mistaken for less-serious conditions, especially in the case of infants and young children.

Genetics plays a significant part in allergy. A person who has two allergic parents or whose family has a history of allergy is 10 times more likely to develop an allergy than is a person from a nonallergic family, and symptoms will usually appear at an earlier age.

Yet people with no trace of an allergic inheritance may also develop allergies, and some members of an allergic family may be completely free of allergic illness throughout their entire lives. Apparently, it is not the allergy itself, but rather, a tendency or susceptibility to allergy, that is passed from parent to child.


Exactly what causes an allergic reaction? Briefly, when a foreign substance first enters the body, it finds its way to the bloodstream. This foreign agent stimulates the production of immunoglobulin E (IgE), a type of antibody specifically produced to combat the invader.

IgE antibodies attach themselves to mast cells, located in parts of the body where foreign substances typically enter, such as respiratory passages, skin surfaces, and the digestive tract. The mast cell–IgE antibody complex can be likened to a mine ready to detonate at the first sign of the allergen's reappearance, even if the allergen does not invade again for many years.

When the same kind of allergen enters the bloodstream again, it triggers the mast cell–IgE antibody complex to release chemicals called mediators from the mast cells. These mediators attack the allergen, but in the process they may damage surrounding tissue. More than 15 mediators with different functions have been identified. The histamines, released in hay fever, are an example of these chemicals. They initiate a local inflammatory response resulting in sneezing and watery, puffy, itchy eyes. The exact symptoms of an allergic reaction depend on the effects of the mediators and in which part of the body the allergen–mast cell–IgE complex exists. Three major types of allergenic substances are inhalants, foods, and skin-contact substances.


Inhalant materials are the major cause of allergic attacks, including asthmatic episodes. The most common is plant pollen, such as that produced by the flowers of ragweed, grass, and trees. These pollens are generally the cause of seasonal hay fever. Usually the allergic person is sensitive to only one type of pollen, but sometimes one type of seasonal allergy leads to another.

Ordinary house dust, formed by the slow deterioration of many different materials in the home, is another inhalant offender. The hair and dander of many animals are allergenic materials, as are feathers, molds, insect sprays, and vegetable fibers. Strong odors and fumes, including cigarette smoke, may bring about a severe attack in an allergic individual.

Foods and liquids.

Food is a common source of allergic attacks. Eggs, milk, nuts, wheat, fish, meats, chocolate, and many other standard foods may produce an allergic reaction when eaten by themselves, or together with other allergenic foods, or as a minor ingredient in an otherwise harmless dish.

Skin contact.

Perhaps the best-known cause of allergic reactions through direct contact is poison ivy, which will cause an itchy, bothersome skin rash, even if a person merely brushes against the plant. But there are many other materials that may cause such a reaction in sensitive individuals, including cosmetics, hair dyes, clothing fabrics, plastics, metals, woods, chemicals, paints and varnishes, and jewelry.

Other allergens.

Medications such as penicillin, aspirin, and sulfa drugs can cause severe allergic reactions whether injected, swallowed, or through mere skin contact. Insect stings and bites, particularly those of the yellow jacket and other wasps and bees, also have been known to cause violent allergic reactions.

These types of allergens produce a systemic (whole-body) reaction, as opposed to a local reaction. Very large amounts of mediator chemicals enter the circulatory system and cause lowered blood pressure and constriction of the small air passages in the lungs. In severe cases, without medical intervention, death may occur within half an hour. In the United States, allergic systemic reaction, called anaphylaxis, accounts for several thousand deaths a year.

The exact role of emotions in allergy, particularly asthma, remains uncertain.


A skilled allergist may correctly diagnose an allergy by the symptoms alone. Often, however, further study is needed.

Medical history.

If the patient had frequent bouts of colic or diaper rash as an infant, or if parents or other close relatives have been treated for allergy, it is very likely that the patient is susceptible to allergy. A complete medical history may also indicate the specific factors in the patient's environment that cause allergic attacks.

Skin test.

A small amount of pollen, egg, or other suspected allergenic material is mixed with a solvent and applied to a scratch in the skin. A different substance is applied to each scratch. If the patient is allergic to one of the materials, a minor reaction will usually appear on the site of the scratch, often within minutes.

Radioallergosorbent test (RAST).

In some cases, a blood test can be used instead of a skin test. A sample of the patient's blood and an allergen are placed together in a test tube. The RAST measures the amounts of specific IgE antibodies present in the patient's blood. High levels of IgE indicate an immune response to the specific allergen.


Once the cause of the allergy has been determined, the simplest and most effective treatment is to avoid all future contact with the offending substance. Unfortunately, this strategy is not always possible. While it is relatively easy to avoid swallowing aspirin, touching poison ivy, or eating allergenic foods, it is considerably more difficult to avoid house dust or ragweed pollen during the height of the pollen season.


When contact with the allergenic material cannot be avoided, the patient's system must be trained to live with it. This is done by means of hyposensitization. The physician first injects an extremely small amount of the offending substance into the patient's system. Gradually, the amount injected is increased. In time, the patient's body builds a tolerance and becomes used to the allergen.

In the case of a seasonal allergy, injections may be given once or twice a week for several months before the start of each pollen season; alternatively, after the first year, the patient may be injected once every two or three weeks on a year-round schedule. At least two full years without any signs of allergy are necessary before hyposensitization can be halted. Even then, symptoms can reappear at any time.

In another type of therapy, the pollen extract in a nasal spray causes the body to produce immunoglobulin G (IgG). This antibody then competes with IgE to block the body's reaction to the allergen. This method will enable patients to administer their own allergy treatments, thus reducing doctor visits and costs.


Great progress has been made in relieving the major symptoms of common allergies. Antihistamine drugs provide temporary relief for minor allergies. Steroid hormones (cortisone and hydrocortisone and their synthetic substitutes) can also be extremely valuable. Both antihistamines and steroids are generally used only for temporary relief until hyposensitization can take effect.

For people who suffer from asthma, beta-agonist bronchodilators widen respiratory passageways by quickly alleviating the shortness of breath that signals an asthma attack. Although providing symptom relief, these drugs do not address the underlying problem. Some studies suggest that asthma patients who rely heavily on inhaled beta-agonist bronchodilators increase their risk of dying, perhaps because opening the airways exposes the lungs to increased concentrations of allergens. Long-term treatment with anti-inflammatory drugs, including inhaled steroids and cromolyn sodium, generally forms the basis of asthma management.

As the mechanism of allergy reaction is better understood, new treatments may arise. Work with conjugated allergens (complexes of allergens with chemicals) suggests that IgE production and the level of IgE in the blood may be controlled. Also, by the early 2000s, researchers were working on new allergy vaccines that combine allergy-triggering substances with synthetic DNA (deoxyribonucleic acid).

The information provided should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies.

Copyright Information: Public domain information with acknowledgement given to the U.S. National Library of Medicine.

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