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In this review you will learn how allergy relates to the immune system. You will begin understanding how and why certain people become allergic. The most common allergic diseases are discussed briefly in this article.

What does an allergy mean?
An allergy refers to an exaggerated reaction by our immune system in response to bodily contact with certain foreign substances. It is exaggerated because these foreign substances are usually seen by the body as harmless and no response occurs in non- allergic people. Allergic people's bodies recognize the foreign substance and one part of the immune system is turned on. Allergy-producing substances are called "allergens." Examples of allergens include pollens, dust mite, molds, danders, and foods. To understand the language of allergy it is important to remember that allergens are substances that are foreign to the body and can cause an allergic reaction in certain people.

When an allergen comes in contact with the body, it causes the immune system to develop an allergic reaction in persons who are allergic to it. When you inappropriately react to allergens that are normally harmless to other people, you are having an allergic reaction and can be referred to as allergic or atopic. Therefore, people who are prone to allergies are said to be allergic or "atopic."

Austrian pediatrician Clemens Pirquet (1874-1929) first used the term allergy. He referred to both immunity that was beneficial and to the harmful hypersensitivity as "allergy." The word allergy is derived from the Greek words "allos," meaning different or changed and "ergos," meaning work or action. Allergy roughly refers to an "altered reaction." The word allergy was first used in 1905 to describe the adverse reactions of children who were given repeated shots of horse serum to fight infection. The following year, the term allergy was proposed to explain this unexpected "changed reactivity."

Allergy Fact
It is estimated that 50 million North Americans are affected by allergic conditions.The cost of allergies in the United States is more than $10 billion dollars yearly.Allergic rhinitis (nasal allergies) affects about 35 million Americans, 6 million of whom are children. Asthma affects 15 million Americans, 5 million of whom are children.The number of cases of asthma has doubled over the last 20 years.

What causes allergies?

To help answer this question, let's look at a common household example. A few months after the new cat arrives in the house, dad begins to have itchy eyes and episodes of sneezing. One of the three children develops coughing and wheezing, especially when the cat comes into her bedroom. The mom and the other two children experience no reaction whatsoever to the presence of the cat. How can we explain this?
The immune system is the body's organized defense mechanism against foreign invaders, particularly infections. Its job is to recognize and react to these foreign substances, which are called antigens. Antigens are substances that are capable of causing the production of antibodies. Antigens may or may not lead to an allergic reaction. Allergens are certain antigens that cause an allergic reaction and the production of IgE.
The aim of the immune system is to mobilize its forces at the site of invasion and destroy the enemy. One of the ways it does this is to create protective proteins called antibodies that are specifically targeted against particular foreign substances. These antibodies, or immunoglobulins (IgG, IgM, IgA, IgD), are protective and help destroy a foreign particle by attaching to its surface, thereby making it easier for other immune cells to destroy it. The allergic person however, develops a specific type of antibody called immunoglobulin E, or IgE, in response to certain normally harmless foreign substances, such as cat dander. To summarize, immunoglobulins are a group of protein molecules that act as antibodies. There are five different types; IgA, IgM, IgG, IgD, and IgE. IgE is the allergy antibody.
(In 1967, the husband and wife team of Kimishige and Teriko Ishizaka detected a previously unrecognized type of immunoglobulin in allergic people. They called it gamma E globulin or IgE.)
In the pet cat example, the dad and the youngest daughter developed IgE antibodies in large amounts that were targeted against the cat allergen, the cat dander. The dad and daughter are now sensitized or prone to develop allergic reactions on subsequent and repeated exposures to cat allergen. Typically, there is a period of "sensitization" ranging from months to years prior to an allergic reaction. Although it might occasionally appear that an allergic reaction has occurred on the first exposure to the allergen, there must have been a prior contact in order for the immune system to be poised to react in this way.
IgE is an antibody that all of us have in small amounts. Allergic persons, however, produce IgE in large quantities. Normally, this antibody is important in protecting us from parasites, but not from cat dander or other allergens. During the sensitization period, cat dander IgE is being overproduced and coats certain potentially explosive cells that contain chemicals. These cells are capable of causing an allergic reaction on subsequent exposures to the dander. This is because the reaction of the cat dander with the dander IgE irritates the cells and leads to the release of various chemicals, including histamine. These chemicals, in turn, cause inflammation and the typical allergic symptoms. This is how the immune system becomes exaggerated and primed to cause an allergic reaction when stimulated by an allergen.
On exposure to cat dander, the mom and the other two children produce other classes of antibodies, none of which cause allergic reactions. In these non-allergic members of the family, the dander particles are eliminated uneventfully by the immune system and the cat has no effect on them.

Figure 1
The Immune System

Foreign Substance
(cat dander, pollen, virus, bacteria)
Normal Immune Response
IgM, IgG, IgA, IgD and various
immune cells respond to attack.

  Exaggerated Immune Response
IgE is overproduced in response to
cat dander, pollens, and other
harmless allergens.
Foreign substance is eliminated.   Subsequent exposure results in an allergic reaction.
Non-Allergic Individual   Allergic Individual
Who is at risk and why?
Allergies can develop at any age, possibly even in the womb. They commonly occur in children but may give rise to symptoms for the first time in adulthood. Asthma may persist in adults while nasal allergies tend to decline in old age.
Why, you may ask, are some people "sensitive" to certain allergens while most are not? Why do allergic persons produce more IgE than those who are non-allergic? The major distinguishing factor appears to be heredity. For some time, it has been known that allergic conditions tend to cluster in families. Your own risk of developing allergies is related to your parents' allergy history. If neither parent is allergic, the chance that you will have allergies is about 15%. If one parent is allergic, your risk increases to 30% and if both are allergic, your risk is greater than 60%.
Although you may inherit the tendency to develop allergies, you may never actually have symptoms. You also do not necessarily inherit the same allergies or the same diseases as your parents. It is unclear what determines which substances will trigger a reaction in an allergic person. Additionally, which diseases might develop or how severe the symptoms might be is unknown.
Another major piece of the allergy puzzle is the environment. It is clear that you must have a genetic tendency and be exposed to an allergen in order to develop an allergy. Additionally, the more intense and repetitive the exposure to an allergen and the earlier in life it occurs, the more likely it is that an allergy will develop.There are other important influences that may conspire to cause allergic conditions. Some of these include smoking, pollution, infection, and hormones.

What are common allergic conditions and their symptoms and signs?

The parts of the body that are prone to react to allergies include the eyes, nose,
lungs, skin, and stomach. Although the various allergic diseases may appear different,
they all result from an exaggerated immune response to foreign substances in sensitive
people. The following brief descriptions will serve as an overview of common allergic

Common symptoms of allergy
Affected organ Symptom
swelling of the nasal mucosa (allergic rhinitis)
Sinuses allergic sinusitis
redness and itching of the conjunctiva (allergic conjunctivitis)
Sneezing, coughing, bronchoconstriction, wheezing and dyspnea, sometimes outright attacks of asthma, in severe cases the airway constricts due to swelling known as laryngeal edema
feeling of fullness, possibly pain, and impaired hearing due to the lack of eustachian tube drainage.
Gastrointestinal tract

Allergic Rhinitis

Allergic rhinitis ("hay fever") is the most common of the allergic diseases and refers to seasonal nasal symptoms that are due to pollens. Year round or perennial allergic rhinitis is usually due to indoor allergens, such as dust mites, animal dander, or molds. It can also be caused by pollens. Symptoms result from the inflammation of the tissues that line the inside of the nose (mucus lining or membranes) after allergens are inhaled. Adjacent areas, such as the ears, sinuses, and throat can also be involved. The most common symptoms include:

In 1819, an English physician, John Bostock, first described hay fever by detailing his own seasonal nasal symptoms, which he called "summer catarrh." The condition was called hay fever because it was thought to be caused by "new hay."


Asthma is a breathing problem that results from the inflammation and spasm of the lung's air passages (bronchial tubes). The inflammation causes a narrowing of the air passages, which limits the flow of air into and out of the lungs. Asthma is most often, but not always, related to allergies. Common symptoms include:

Allergic Eyes

Allergic eyes (allergic conjunctivitis) is inflammation of the tissue layers (membranes) that cover the surface of the eyeball and the undersurface of the eyelid. The inflammation occurs as a result of an allergic reaction and may produce the following symptoms:

  • Redness under the lids and of the eye overall
  • Watery, itchy eyes
  • Swelling of the membranes

Allergic eczema (atopic dermatitis) is an allergic rash that is usually not caused by skin contact with an allergen. This condition is commonly associated with allergic rhinitis or asthma and features the following symptoms:

  • Itching, redness, and or dryness of the skin
  • Rash on the face, especially children
  • Rash around the eyes, in the elbow creases, and behind the knees, especially in older children and adults (rash can be on the trunk of the body)
Hives (urticaria) are skin reactions that appear as itchy swellings and can occur on any part of the body. Hives can be caused by an allergic reaction, such as to a food or medication, but they also may occur in non-allergic people. Typical hive symptoms are:
Raised red welts
Intense itching

Allergic shock (anaphylaxis or anaphylactic shock) is a life-threatening allergic reaction that can affect a number of organs at the same time. This response typically occurs when the allergen is eaten (for example, foods) or injected (for example, a bee sting). Some or all of the following symptoms may occur:

Shock refers to the insufficient circulation of blood to the body's tissues. Shock is most commonly caused by blood loss or an infection. Allergic shock is caused by dilated and "leaky" blood vessels, which result in a drop in blood pressure.

Where are allergens?
We have seen that allergens are special types of antigens that cause allergic reactions. The symptoms and diseases that result depend largely on the route of entry and level of exposure to the allergens. The chemical structure of allergens affects the route of exposure. Airborne pollens, for example, will have little effect on the skin. They are easily inhaled and will thus cause more nasal and lung symptoms and limited skin symptoms. When allergens are swallowed or injected they may travel to other parts of the body and provoke symptoms that are remote from their point of entry. For example, allergens in foods may prompt the release of mediators in the skin and cause hives.
We will assume that allergens are defined as: the source of the allergy producing substance (for example, cat), the substance itself (cat dander), or the specific proteins that provoke the immune response (for example, Feld1). Feld1, from the Felis domesticus (the domesticated cat), is the most important chemical allergen in cat dander.
Allergens may be inhaled, ingested (eaten or swallowed), applied to the skin, or injected into the body either as a medication or inadvertently by an insect sting.
In the Air We Breathe
Breathing can be hazardous if you are allergic. Aside from oxygen, the air contains a wide variety of particles; some toxic, some infectious, and some "innocuous," including allergens. The usual diseases that result from airborne allergens are hay fever, asthma, and conjunctivitis. The following allergens are usually harmless, but can trigger allergic reactions when inhaled by sensitized individuals.
Pollens: trees, grasses, and/or weeds
Dust mites
Animal proteins: dander, skin, and/or urine
Mold spores
Insect parts: cockroaches

In What We Ingest

When foods or medications are ingested, allergens may gain access to the blood stream and become attached to specific IgE on cells in remote sites such as the skin or nasal membranes. The ability of allergens to travel explains how symptoms can occur in areas other than the gastrointestinal tract. Food allergy reactions may begin with tongue or throat swelling and may be followed by tingling, nausea, diarrhea, or stomach cramps. Nasal breathing difficulties or skin reactions may also be seen. The two main allergen groups that are ingested are:

  • Foods
  • Drugs (when taken by mouth): for example, antibiotics and aspirin

Allergy Assist The most common foods that cause allergic reactions are cow's milk, fish, shellfish, eggs, peanuts, tree nuts, soy, and wheat.

Touching Our Skin

Allergic contact dermatitis is an inflammation of the skin that is caused by a local allergic reaction. The majority of these localized skin reactions do not involve IgE, but are caused by cells of inflammation. The rash produced is similar to that of a poison ivy rash. It should be noted that when some allergens (for example, latex) come into contact with the skin, they are absorbed by the skin and can also potentially cause reactions throughout the body, not just the skin. For most people, however, the skin is a formidable barrier that can be only locally affected. Examples of allergic contact dermatitis include:

  • Latex (causes IgE and non-IgE reactions)
  • Plants (poison ivy and oak)
  • Dyes
  • Chemicals
  • Metals (nickel)
  • Cosmetics

Allergic contact dermatitis does not involve IgE antibody, but involves cells of the immune system which are programmed to react when triggered by a sensitizing allergen. Touching or rubbing a substance to which you were previously sensitized can trigger a skin rash.

Injected into Our Body

The most severe reactions can occur when allergens are injected into the body and gain direct access to the blood stream. This access carries the risk of a generalized reaction, such as anaphylaxis, which can be life-threatening. The following are commonly injected allergens that can cause severe allergic reactions:

Allergy At A Glance
  • Allergy involves an exaggerated response of the immune system.
  • The immune system is the body's organized defense mechanism against foreign invaders, particularly infections.
  • Allergens are substances that are foreign to the body and can cause an allergic reaction.
  • IgE is the allergy antibody.
  • Allergies can develop at any age.
  • Your risk of developing allergies is related to your parents' allergy history.


Skin testing

Skin testing on arm

Skin testing on back
For assessing the presence of allergen-specific IgE antibodies, allergy skin testing is preferred over blood allergy tests because it is more sensitive and specific, simpler to use, and less expensive. Skin testing is also known as "puncture testing" and "prick testing" due to the series of tiny puncture or pricks made into the patient's skin. Small amounts of suspected allergens and/or their extracts (pollen, grass, mite proteins, peanut extract, etc.) are introduced to sites on the skin marked with pen or dye (the ink/dye should be carefully selected, lest it cause an allergic response itself). A small plastic or metal device is used to puncture or prick the skin. Sometimes, the allergens are injected "intradermally" into the patient's skin, with a needle and syringe. Common areas for testing include the inside forearm and the back. If the patient is allergic to the substance, then a visible inflammatory reaction will usually occur within 30 minutes. This response will range from slight reddening of the skin to a full-blown hive (called "wheal and flare") in more sensitive patients. Interpretation of the results of the skin prick test is normally done by allergists on a scale of severity, with +/- meaning borderline reactivity, and 4+ being a large reaction. Increasingly, allergists are measuring and recording the diameter of the wheal and flare reaction. Interpretation by well-trained allergists is often guided by relevant literature. Some patients may believe they have determined their own allergic sensitivity from observation, but a skin test has been shown to be much better than patient observation to detect allergy.

If a serious life threatening anaphylactic reaction has brought a patient in for evaluation, some allergists will prefer an initial blood test prior to performing the skin prick test. Skin tests may not be an option if the patient has widespread skin disease or has taken antihistamines sometime the last several days.

Blood testing

Various blood allergy testing methods are also available for detecting allergy to specific substances. This kind of testing measures a "total IgE level" - an estimate of IgE contained within the patient's serum. This can be determined through the use of radiometric and colormetric immunoassays. Radiometric assays include the radioallergosorbent test (RAST) test method, which uses IgE-binding (anti-IgE) antibodies labeled with radioactive isotopes for quantifying the levels of IgE antibody in the blood. Other newer methods use colorimetric or fluorometric technology in the place of radioactive isotopes. Some "screening" test methods are intended to provide qualitative test results, giving a "yes" or "no" answer in patients with suspected allergic sensitization. One such method has a sensitivity of about 70.8% and a positive predictive value of 72.6% according to a large study.
A low total IgE level is not adequate to rule out sensitization to commonly inhaled allergens. Statistical methods, such as ROC curves, predictive value calculations, and likelihood ratios have been used to examine the relationship of various testing methods to each other. These methods have shown that patients with a high total IgE have a high probability of allergic sensitization, but further investigation with specific allergy tests for a carefully chosen allergens is often warranted.
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