Allergens and Allergies Basics

Allergens and Allergies

Reviewed By:
Norman Klein, M.D., FAAAAI


An allergen is any substance that the body mistakenly perceives as a threat, triggering a specific chain of events called an allergic cascade. The immune system protects the body from harmful foreign substances. Sometimes, it mistakenly identifies a harmless allergen as a threat. The resulting allergic reaction causes symptoms, such as sneezing, coughing, runny nose, sore throat, diarrhea, vomiting, itching and hives. More severe reactions can result in lower blood pressure, difficulty breathing and, in some cases, death.

Most allergies fall into one or more of the following categories:

  • Airborne substance allergies (e.g., pollen, dust, mold)
  • Food allergies
  • Animal or insect allergies
  • Cosmetic or chemical allergies
  • Drug or medication allergies (over-the-counter or prescription)

Allergy management usually includes the use of medications to prevent or relieve allergy symptoms. These include:

  • Antihistamines (prevent the release of symptom-causing chemicals into the body)
  • Decongestants (reduce nasal and chest congestion)
  • Corticosteroids and NSAIDs (reduce inflammation)
  • Bronchodilators (open breathing passages)
  • Other medications (e.g., leukotriene modifiers, mast cell stabilizers)

The most effective way to treat an allergy is to avoid the allergen. However, this can be difficult with certain allergens. For instance, people with food allergies need to avoid not only the problem food, but all products that contain that food or components of that food. Individuals with allergic rhinitis (hay fever) often find it difficult to avoid the airborne pollens that are present in the outdoor air at some times of the year.

Depending on the allergen, there are tools to help people avoid contact with an allergen. These include:

  • Allergy index. A measure of the number of people affected by pollen in a certain region. Based on a scale of 1 to 10, it takes into account both the amount of pollen present in the air and how sensitive most people are to the pollen types present.

  • Pollen and mold report. Describes the current amount of airborne allergens present in the air. Results are presented in grains per cubic meter of air.

  • Food labels. A list all of a food’s ingredients on the packaging of the food, required in the United States by the Food and Drug Administration. This is a very valuable resource for individuals with food allergies. Because of the complexity of today’s food manufacturing and production, food labels are often the only way to tell if a food includes allergy-causing ingredients.

Asthma, a condition in which the airways become blocked or narrowed, can be exacerbated by contact with allergens. Asthma attacks are triggered by many different environmental factors including allergens, exercise and cold air. An allergic response to allergens such as pollen, dander or certain foods can cause the airways to inflame and constrict, leading to an asthma attack. Individuals having an asthma attack often experience shortness of breath, breathing difficultyand cough. 

About allergens and allergies

Allergies are exaggerated reactions of the immune system to certain foreign invaders that it mistakes as a threat to the body. When an antigen enters the body, the immune system works to identify whether that substance is dangerous. With an allergy, the immune system incorrectly identifies a harmless substance (e.g. pollen, animal dander, eggs, milk) as dangerous.

This misidentification kicks off a series of events known as an allergic cascade that results in an allergic reaction and uncomfortable allergy symptoms (e.g. sneezing, coughing, runny nose, sore throat, diarrhea, vomiting, itching and hives). Those substances a person is sensitive to are called allergens. 

Allergens may be inhaled (e.g., pollen, dander), come in direct contact with the skin or mucous membranes (e.g., latex, poison ivy, oak and sumac), be injected (e.g., insect sting), or be ingested (e.g., peanuts, eggs).

There are three basic phases of an allergic cascade:

  1. The immune system first encounters a substance that it decides is threatening and begins to produce a specific antibody called immunoglobulin E (IgE) that allows it to recognize the allergen on subsequent encounters. In other words, the person becomes sensitized to the allergen.

  2. The person encounters the allergen again and IgE antibodies trigger the release of histamines and other chemicals into the bloodstream.

  3. Allergy symptoms appear. They may be localized (only in the area where these chemicals were first released) or systemic (throughout the entire body). Chemicals released during an allergic reaction mainly affect the blood vessels, mucous glands and bronchial tubes. Symptoms usually involve tissue swelling and inflammation.

The most common allergies fall into one or more of the following categories:

  • Indoor allergies (e.g., dust, mold, dander)
  • Outdoor allergies (e.g., pollen, plants, sun, cold)
  • Food allergies
  • Animal allergies or insect sting allergies
  • Cosmetic allergies or other chemical allergies
  • Drug allergies (over-the-counter or prescription medications)

The most severe kind of allergic response an individual can have is anaphylaxis. This is a severe and potentially life-threatening reaction that occurs in certain highly-sensitive individuals. The condition is usually caused by an allergic reaction to certain foods, insect stings or drugs.

Symptoms of anaphylaxis include shortness of breath, tightness in the chest, swelling (of the lips and face), vomiting, diarrhea and lowered blood pressure. Individuals who are susceptible to anaphylaxis should carry an epinephrine injection with them, which can quickly reverse the symptoms.

Many types of allergens are triggers for people with asthma, a condition in which the airways are blocked or constricted. Allergens often cause the body to react with inflammations that further constrict airways, provoking an asthma attack in susceptible individuals. Asthma symptoms may include shortness of breath, cough and difficulty breathing.

Some people will find they are more sensitive to allergies or asthma attacks after exercising. This is because exercise – particularly in cold weather – causes the body’s temperature to rise and tiny muscles surrounding the air passageways to twitch. This restricts airflow slightly, even for several minutes after the exercise is finished.

If a person has an allergic reaction when they are already suffering from slightly impaired breathing and a higher body temperature, the results can be a much more serious event. Some people are allergic to certain foods only when they have recently exercised.

Types of allergens

Some of the most common allergens include:

  • Pollen. Given off seasonally by trees, grass and weeds, pollen is responsible for triggering most cases of allergic rhinitis, also known as hay fever. The plant most commonly associated with pollen allergies is ragweed.
  • Molds. Often found indoors where the humidity is high, mold can easily become airborne and trigger cases of allergic rhinitis year-round.
  • Animal dander. Small scales or flakes of dead skin from animals (usually cats and dogs) are responsible for many allergic rhinitis symptoms.

  • Dust Mites. These microscopic insects live in the dust found indoors and often trigger allergic rhinitis symptoms.

  • Eggs. One of the most common types of food allergy, egg allergies are common in children, but are often outgrown.

  • Milk. This type of food allergy involves a reaction to one or more of the proteins found in cow’s milk or dairy products. Like egg allergies, milk allergies are often outgrown.

  • Wheat. The proteins found in wheat can trigger an immune system reaction in some people.

  • Seafood. This type of food allergy is characterized by an allergic reaction to ingested fish or shellfish.

  • Peanuts and tree nuts. These food allergies are triggered by peanuts, tree nuts (e.g., walnuts, almonds) or any product that uses peanuts or tree nuts as an ingredient. Highly sensitive individuals may react to even trace amounts picked up from utensils, surfaces or packaging machinery. Reactions can include anaphylactic shock.

  • Soy. Most commonly found in infants, this type of allergy involves an immune response to soybeans, as well as foods and other products (e.g., some inks, soaps and dyes) that contain soy.

  • Insect stings or bites. Some people experience serious or life-threatening reactions (anaphylaxis) after being bitten or stung by an insect.

  • Penicillin. This common type of drug allergy can cause a range of symptoms including anaphylactic shock. The drug penicillin is commonly used to treat infections.

  • Latex. Sensitivity to the sap in rubber trees causes some people to be allergic to latex products (made from rubber tree sap). Reactions to latex usually occur when skin-to-skin contact is made, but they are also possible through inhalation of small latex particles.

  • Metals. Metals such as nickel, cobalt and chromate can trigger allergic reactions.

  • Cosmetics. People can become allergic to any type of cosmetic or chemicals used in them. Cosmetics include makeup, skin-care creams, lotions, powders, sprays, perfumes, fingernail polishes, permanent waves, hair colors, deodorants, baby products, bath oils, bubble baths and mouthwashes. Fragrances cause more allergic reactions than any other cosmetic ingredient.

  • Chemicals. Some people have allergic reactions to chemicals within household cleansing products. However, reactions can also occur in non-allergic people because many chemicals are irritants. Reactions can be related to contact (skin rash) or inhalation (wheezing) of these chemicals.
  • Poisonous plants. Itchy bumps can form on the skin of some people sensitive to these plants.
  • Sun, heat or cold. A variety of rashes may appear in sensitive people after exposure to sunlight or high temperatures. Conversely, some people can be allergic to cold temperatures.

Signs and symptoms of allergies

Symptoms of an allergic reaction can affect many different parts of the body as described below:


  • Shortness of breath or difficulty breathing
  • Congestion
  • Chest pain or tightness
  • Wheezing
  • Coughing

Nose and sinuses

  • Stuffiness
  • Runny nose
  • Congestion
  • Sneezing


  • Itchiness
  • Swelling
  • Hives
  • Redness
  • Rash


  • Nausea
  • Vomiting
  • Diarrhea
  • Cramps


  • Itchiness or burning
  • Swelling or puffiness
  • Redness
  • Watering
  • Dark under eyes


  • Swelling in the face, lips, tongue or throat (angioedema)
  • Fatigue
  • Uterine cramps

People who believe they are experiencing anaphylaxis, a severe and potentially life-threatening reaction that occurs in certain highly-sensitive individuals, require immediate medical attention. Individuals should use epinephrine at the first sign of an anaphylactic reaction and then call an ambulance for additional medical treatment. Symptoms of anaphylaxis include:

  • Shortness of breath
  • Tightness in the chest
  • Swelling of the lips, face, throat or tongue
  • Vomiting
  • Diarrhea
  • Lowered blood pressure
  • Feeling faint or dizzy

Diagnosis methods for allergies

The first step in diagnosing any condition is likely to be a physical examination. When allergies are suspected, the physician will pay close attention to the patient’s ears, eyes, nose, throat, chest and skin. A pulmonary function test may be used to measure how effectively the patient exhales air from the lungs. X-rays of the lungs or sinuses may also be performed.  

A detailed medical history will also be taken. The physician will inquire as to whether any allergies have been diagnosed in the past. This is significant because a person who is allergic to one substance has a greater chance of being allergic to another substance. In addition, different substances may be chemically similar, triggering an allergic reaction. A list of current medications will be collected as well.

Family medical history is also a factor. If one parent has allergies, there is a 50 percent chance of a child developing allergies, according to the American Academy of Allergy, Asthma and Immunology. If both parents have allergies, this risk jumps to 75 percent.

Physicians will also ask questions about the patient’s lifestyle at home, work or school. This information will further help the physician pinpoint the allergens causing the patient’s allergic reactions.

The primary treatment for any allergy is avoidance – refraining from any contact with the triggering allergen. Because of this, the diagnosis of an allergy involves identifying the specific allergen that triggers the patient’s allergic reaction. Tests that may be used for this purpose include:

  • Skin test. This test introduces various allergens to the patient’s skin in specific areas. If redness or swelling occurs in an area, it usually indicates that the patient is allergic to that substance. This type of test does not provide results that are 100 percent accurate and is not usually appropriate for people with certain skin conditions.
  • RAST (radioallergosorbent test). This blood test allows a laboratory to directly test a blood sample for antibodies that correspond to specific allergens. While less sensitive than a skin test, it can be used on individuals who have reactions too severe to risk the exposure to the allergen required for skin testing. A RAST test is often used to determine sensitivity to food allergies.

  • Elimination Diet. Used in the diagnosis of food allergies, this involves removing suspect foods from a patient’s diet to see if the allergic reactions persist. This trial-and-error approach often takes weeks for results, but it can be effective at pinpointing a problem food.

  • Oral food challenge. This is considered the most accurate way of determining the cause of a food allergy because it supplies the most convincing results. Different foods are placed within capsules to hide their identity. The patient consumes the capsules and the physician looks for signs of an allergic reaction. This type of test is time-consuming and difficult. It is often reserved to confirm suspicions that a patient’s symptoms are not caused by a food allergy.

Some people turn to complementary and alternative medicine (CAM) allergy tests as an alternative approach to diagnosing allergies. These tests are often unproven and have not shown efficacy in clinical trials. A patient may want to seek a second opinion if a physician suggests one of these tests:

  • Cytotoxicity blood test
  • Applied kinesiology
  • Subcutaneous or sublingual provocation testing
  • Neutralization testing
  • Electroacupuncture biofeedback
  • Urine autoinjection
  • Skin titration (Rinkel method)
  • Candidiasis allergy theory
  • Basophil histamine release
  • ELISA/ACT (not to be confused with ELISA, which is often used to test for autoimmune diseases and sometimes for allergies)

Treatment options for allergies

Allergy treatment begins with avoidance – completely avoiding the allergen known to trigger an allergic reaction. Since avoidance is not always possible, a number of other strategies may be necessary to treat allergic conditions.

The majority of allergy treatments are designed to ease symptoms of an allergic reaction that has already occurred. Allergy medications can be administered to an individual through a number of methods, including pills, inhalers, nasal sprays, eye drops, nebulizers and topical creams.

The following types of medication are used to treat allergy symptoms:

  • Antihistamines. Relieve allergy symptoms such as sneezing, runny nose and itchy and watery eyes. They work by interfering with the histamines released into the bloodstream when a person encounters an allergen. Histamines are directly responsible for causing many kinds of allergy symptoms, and blocking them reduces the severity of an allergic reaction. While antihistamines can be taken to treat symptoms that are already present, the drugs are most effective when taken prior to coming into contact with an allergen.

  • Decongestants. Reduce nasal congestion, chest congestion, swelling and redness. Decongestants are effective at constricting swollen nasal tissue, which prevents fluid and mucus from forming.

  • Corticosteroids and NSAIDs. Reduce the inflammation associated with symptoms such as nasal stuffiness, sneezing and runny nose. Corticosteroids work by inhibiting allergic reactions and reducing nasal swelling and mucus secretions. NSAIDs (nonsteroidal anti-inflammatory drugs) are medications used to reduce pain and inflammation associated with allergy symptoms without the use of corticosteroids.

  • Bronchodilators. Open breathing passages by relaxing tightened muscles. These drugs are also effective at loosening mucus in the lungs, which can then be more easily coughed out. Bronchodilators are often used to quickly treat symptoms of wheezing, chest tightness and difficulty breathing.

  • Other medications. Other types of medications include mast cell stabilizers, leukotriene modifiers, anti-IgE antibodies, anticholinergics, vitamin C and nasal saline solutions.

There is currently only one treatment designed to help a person with a known allergy to overcome their sensitivity to a certain allergen – immunotherapy (or allergy shots). Patients undergo a series of shots over a period of weeks, months or years with each shot containing more of the allergen to which the person is sensitive. Through this exposure, the patient’s tolerance to the allergen increases and the body no longer reacts to that allergen in the same way. However, not everyone responds to this treatment and it is not effective at treating some types of allergies (e.g., food allergies).

Several types of allergy treatment are commonly used, though they have not been tested through strict scientific criteria, including homeopathy, acupuncture and traditional Chinese medicine. These alternate allergy treatments are often not accepted or used by licensed physicians or other licensed healthcare providers. People should discuss any alternative treatments with their physician before trying them.

Prevention methods for allergies

The majority of allergy treatments are designed to ease symptoms of an allergic reaction that has already occurred. There are some treatments, however, designed to prevent allergic reactions in people with known allergies. The goal of such treatments is one or more of the following:

  • Prevent the onset of symptoms
  • Lessen the frequency of reactions
  • Lessen the severity of symptoms resulting from a reaction

The only treatment that can completely prevent an allergic reaction from occurring is the total avoidance of any known allergens (e.g., pollen, dust, pet dander). However, avoiding an allergen is often difficult when the allergen is constantly present or an individual is highly sensitive.

One resource that is very useful to people trying to avoid the airborne allergens common to allergic rhinitis (hay fever) is the pollen and mold report. This report describes the current amount of airborne allergens present in grains per cubic meter of air. Individuals who are sensitive to airborne allergens can use the report to determine when they should remain indoors. The report is divided into four separate counts: tree pollen, grass pollen, weed pollen and mold spores. 

The allergy index is another useful tool. It is a measure of the number of people affected by pollen in a certain region. Based on a scale of 1 to 10, it takes into account both the amount of pollen present in the air and how sensitive most people are to the pollen types present.

Individuals with food allergies should learn to always check food ingredient labels for problematic foods. Oftentimes what seems like a harmless food will actually contain unexpected substances. For example, some types of beer and wine contain egg proteins, which can be dangerous to people with an egg allergy. People with food allergies must also learn what other names their problem food might be listed as. For example, individuals with an egg allergy must avoid extracts such as albumin, globulin and livetin with equal care.

Allergy shots are the only treatment currently available for preventing allergy symptoms over a long period of time. By using this method, also known as immunotherapy, an individual can become gradually desensitized to a specific allergen. 

While many medications are used to treat the symptoms of allergies, some medications can be taken prior to contact with an allergen to prevent or reduce symptoms. These include:

  • Leukotriene modifiers. Can be taken preventatively to reduce or completely prevent some allergy symptoms. These medications disrupt the function of leukotrienes (involved with the constriction of airways in the lungs). The immune system produces leukotrienes during an allergic reaction.

  • Antihistamines. Can lessen the severity of an allergic reaction by disrupting the allergic cascade. When antihistamines are present in the bloodstream during an allergic reaction, histamines are released, but are blocked from carrying out their normal functions. This means that the symptoms usually caused by histamines, such as itchiness and runny nose, are lessened or prevented. Because they can effectively prevent allergy symptoms, some people take antihistamines on a daily basis.

  • Corticosteroids. Can be taken preventively to reduce or prevent allergy symptoms by counteracting swelling, inflammation and mucus secretions. For the best results, corticosteroids should be taken every day with regularity, and the drug must remain in the body for at least a week to be effective.

The use of air filters can be very effective at reducing the number of airborne allergens in an enclosed space. High-efficiency particulate air (HEPA) filters are one of the most highly effective kinds of filter currently available. By forcing air through screens with microscopic pores, HEPA filters are capable of filtering out at least 90 percent of all particles larger than 0.3 microns, which prevents their release back into the air. HEPA filters can help to substantially diminish the amount of pollen, airborne mold and dander found in a room.

Questions for your doctor

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about allergens and allergy basics:

  1. Do my symptoms indicate allergies?
  2. What tests will you use to determine the cause of my allergic reactions?
  3. What am I allergic to?
  4. Are my allergies dangerous?
  5. Will I experience the same symptoms every time I have a reaction, or will some reactions be more severe?
  6. What are my treatment options?
  7. Am I a candidate for allergy shots?
  8. Will I have to take medication every day or just when I develop symptoms?
  9. How can I avoid contact with allergens?
  10. Are my children more likely to have allergies because I have the condition?
  11. How will having allergies affect my asthma?
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