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Allergy Guide

Medically reviewed by L. Anderson, PharmD. Last updated on Jul 18, 2016.

What is an allergy?

An allergy is an exaggerated immune response or reaction to substances that are generally not considered harmful. You can be allergic to pollen (see hay fever), dust, mold, food and certain medications, for example. Allergic reactions can also be caused by insect bites, jewelry, cosmetics, and almost any substance that contacts the body. Some people have allergic-type reactions to hot or cold temperatures, sunlight, or other physical stimuli, such as friction (rubbing or vigorously stroking the skin).

Some disorders may also be associated with allergies. These include eczema and asthma, among others.

What causes allergies and who is at risk?

An allergy is caused by an oversensitive immune system, which leads to a misdirected immune response. The immune system normally protects the body against harmful substances, such as bacteria and viruses. When an allergic reaction occurs, it is a result of the immune system reacting to substances (allergens) that are generally harmless and in most people do not cause an immune response.

In a person with allergies, the first exposure to the allergen triggers the immune system to recognize the substance. Any exposure after that will usually result in symptoms. When an allergen enters the body of a person with a sensitized immune system, histamine and other chemicals are released by certain cells. This causes itching, swelling, mucus production, muscle spasms, hives, rashes, and other symptoms. These can vary in severity from person to person. Most people have symptoms that cause discomfort without being life-threatening. A few people have life-threatening reactions (called anaphylaxis).

The type of symptoms experienced is partly due to where the allergen is found in the body. For example, inhaled allergens such as pollen often cause nasal congestion, an itchy nose and throat, mucus production, cough, or wheezing. A food allergen can cause nausea, vomiting, abdominal pain, cramping, diarrhea, or a severe, life-threatening reaction. Allergies to plants often cause skin rash. Drug allergies usually involve the whole body.

Allergies are relatively common. Both hereditary and environmental factors have been found to play a role in their development.


Allergy symptoms vary depending on what is causing the reaction and the part of the body where the reaction occurs. Symptoms can include:

  • runny nose
  • tearing, burning, swollen or itchy eyes
  • red eyes, conjunctivitis
  • itchy nose, mouth, throat, skin, or any other area
  • wheezing
  • coughing
  • difficulty breathing
  • hives (skin wheals)
  • skin rashes
  • stomach cramps
  • vomiting
  • diarrhea
  • headache


The history of your symptoms is important in diagnosing all allergies, including whether the symptoms vary according to time of day, season, exposure to pets and other potential allergens, and diet changes. Severe reactions often develop very quickly after exposure, such as eating nuts or getting stung by an insect.

Allergy testing may be required to determine if your symptoms are an actual allergy or caused by other problems. For example, eating contaminated food (food poisoning) may cause symptoms that resemble food allergies. Some medications (such as aspirin, ampicillin, and others) can produce non-allergic reactions, including rashes, that resemble drug allergies but are not true allergies.

Tests that may reveal the specific allergens include:

  • Skin testing -- the most common method of allergy testing. This may include intradermal, scratch, patch, or other tests. Skin testing may even be an option for young children and infants, depending on the circumstances.
  • Blood test -- also called RAST (radioallergosorbent), this measures the levels of allergy antibody, IgE, produced when your blood is mixed with a series of allergens in a laboratory. If you are allergic to a substance, the IgE levels may increase in the blood sample. The blood test may be used if you have existing skin problems like eczema, if you're on medications that are long-acting or that you cannot stop taking, if you have a history of anaphylaxis, or if you prefer not to have a skin test.
  • "Use" or "elimination" tests -- suspected items are eliminated and/or introduced while the person is observed for response to the substance. This is often used to check for food or medication allergies.
  • Eyelid -- Occasionally, the suspected allergen is dissolved and dropped onto the lining of the lower eyelid (conjunctiva) as a means of testing for allergies. (This test should only be done by a physician, never the patient, since it can be harmful if not done properly.)
  • Reaction to physical stimuli -- heat, cold, or another stimulant is applied and the patient is observed to see whether they have an allergic response.

Other tests that may reveal allergies include:

  • Antibody/immunoglobulin (particularly IgE) levels -- when these are elevated, it indicates a "primed" immune system.
  • CBC -- may reveal an increase in eosinophils.
  • Complement levels -- may be abnormal.

Call your doctor if:

Call for an appointment with your health care provider if severe symptoms of allergy occur, if previously successful treatment has become ineffective, or if symptoms appear severe or do not respond to treatment.

Treatment Options

The goal is to reduce the symptoms caused by inflammation of the affected tissues. Of course, the best "treatment" is to avoid what causes your allergies in the first place. It may be impossible to completely avoid everything you are allergic to, but you can often take steps to reduce your exposure. This is especially important for food and drug allergies.

Medications that can be used to treat allergies include the following:

The most appropriate medication depends on the type and severity of symptoms. Specific illnesses that are caused by allergies (such as asthma, hay fever , and eczema) may require other treatments.

Allergy shots (immunotherapy) are occasionally recommended if the allergen cannot be avoided and symptoms are hard to control. Regular injections of the allergen are given, with each dose slightly larger than the previous dose. Allergy shots keep your body from over-reacting to the allergen. They do not work for everybody and require frequent visits to your doctor.

In April 2014, the FDA approved Oralair (Sweet Vernal, Orchard, Perennial Rye, Timothy and Kentucky Blue Grass Mixed Pollens Allergenic Extract) to treat allergic rhinitis that is caused by certain grass pollens in people ages 10 through 65 years. Oralair is the first sublingual (under the tongue) allergen extract approved in the U.S. The first dose of Oralair is given in the clinic; after the first dose it can be taken at home. People affected by hay fever may suffer from repetitive sneezing, nasal itching, runny nose, nasal congestion, and itchy and watery eyes and may need allergy shots; for those with certain grass allergies Oralair may be an alternative.

Severe reactions (anaphylaxis) require epinephrine, which can be life saving when administered soon after exposure by patients themselves, caregivers, or by-standers. To learn more about EpiPen alternatives, see the slideshow: EpiPen Costs and Alternatives: What Are Your Best Options?

  • Short-acting antihistamines - these "first generation antihistamines" are non-prescription and relieve mild-to-moderate allergy symptoms but they can also cause drowsiness. In addition, these antihistamines can blunt learning in children (even in the absence of drowsiness). Examples of shorter-acting antihistamines include diphenhydramine (Benadryl), chlorpheniramine and brompheniramine. These antihistamines are available in generic or store brands and are inexpensive.
  • Longer-acting antihistamines - cause less drowsiness and can be equally effective; usually they do not interfere with learning. These medications, which do not require a prescription, include fexofenadine (Allegra), cetirizine (Zyrtec), and loratadine (Claritin). Cetirizine may be most likely to lead to drowsiness, and if this occurs, it can be taken at bedtime. Loratadine causes either no drowsiness or much less drowsiness and does not affect learning in children. All of these "second generation antihistamines" are available in once-a-day dosing and can be bought at the pharmacy in generic or store brands to save money. Dosing recommendations for children are listed on the OTC Drug Facts Label; if you have questions check with your pharmacist or physician.
  • Nasal corticosteroid sprays are very effective and safe for people with symptoms not relieved by antihistamines alone. These prescription medications include fluticasone (Flonase), mometasone (Nasonex), and triamcinolone (Nasacort AQ). An over-the-counter version of triamcinolone (Nasacort Allergy) was approved in October 2013 and is available without a prescription; this option could save patients quite a bit of money.
  • Decongestants may also be helpful in reducing symptoms such as nasal congestion. Nasal spray decongestants should not be used for more than several days because they can cause a "rebound" effect and make the congestion worse. Decongestants in pill form do not cause this effect.
  • Cromolyn sodium is available as a nasal spray (Nasalcrom) for treating hay fever. Eye drop versions of cromolyn sodium and antihistamines are available for itchy, bloodshot eyes.
  • Leukotriene inhibitors -- montelukast (Singulair) and zafirlukast (Accolate) are oral prescription medicines approved to help control asthma and to help relieve the symptoms of seasonal allergies.

See Also

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.