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Medically reviewed by Last updated on Feb 9, 2024.

What is an allergy?

Harvard Health Publishing

An allergy is an immune system reaction to something that is normally harmless. The reaction to the allergen (allergy-triggering substance) results in the release of histamine and other body chemicals. These chemicals cause the symptoms of allergies, which are usually mild but annoying. Examples include the runny nose of hay fever (allergic rhinitis) or the itchy rash of poison ivy.

However, in some cases, the symptoms can be much worse and involve the entire body. Anaphylaxis is the most severe allergic reaction. In anaphylaxis, these immune chemicals cause serious skin symptoms, such as hives and swelling, as well as severe breathing problems and very low blood pressure.

Allergic reactions can be triggered by a wide variety of allergens. Common ones include:

Symptoms of allergies

Symptoms depend on the type of allergic reaction. Pollen and pet dander allergies typically cause

Nasal congestion can lead to mouth breathing, and the dripping of excess mucus from the nose into the throat can cause persistent cough and sore throat.

The most common symptom of a drug allergy is a skin rash. If you have been exposed to the drug before, the rash may start quickly, within the first day or two after taking the drug. The reaction can also be delayed and can occur as late as 8 to 10 days after starting the drug. It is even possible to develop the rash after you have finished a one-week course of medication. When this does occur, it is usually related to an antibiotic.

Symptoms of anaphylaxis usually occur within seconds to minutes of exposure to the allergen, but symptoms can sometimes be delayed for several hours. Symptoms range from mild to very severe. They can include:

Diagnosing an allergy

The doctor will ask about your symptoms, including when and where you have them, and about your exposure to any of the common allergens that trigger reactions.

Most often, your doctor will recognize an allergic reaction based on your symptoms, medical history, and the results of a physical examination. However, sometimes the symptoms and physical exam findings are less typical. In these cases, allergy skin testing and blood tests can help.

To identify the specific allergen, your doctor may refer you to an allergist, who can do skin testing. In skin testing, a small amount of a specific allergen is scratched, pricked, or injected into the skin. The allergist then monitors your skin for 10 to 20 minutes to see if a reaction develops.

Blood tests can also be done to measure the number of eosinophils (a type of white blood cell that increases in number during allergy seasons) or the levels of IgE, an allergy-inducing antibody. An elevated eosinophil count or IgE level tells the doctor that there is an allergic response, while skin tests provide more specific information about what the person is allergic to.

Expected duration of an allergy

Allergy symptoms can persist year-round, occur only during certain seasons, or happen only when there is exposure to certain allergens. For many, allergy symptoms tend to diminish with age.

With early and appropriate treatment of an anaphylactic reaction, symptoms improve quickly within a few hours. If a person has already developed the more serious symptoms and any related dangerous conditions, it may take a few days to fully recover after treatment. If untreated, anaphylaxis can cause death within minutes to hours.

Preventing allergies

You can help prevent allergic reactions by reducing your exposure to the suspected allergens. For example, to help reduce exposure to outdoor allergens:

You can prevent anaphylaxis by avoiding the allergens that trigger your symptoms. For example, people with food allergies should always check the list of ingredients on food labels, and they should always ask the waiter or waitress to check with the chef about food ingredients before eating at a restaurant. If you are allergic to bee stings, you should limit gardening and lawn mowing, and you should not use perfumes hair sprays, or wear bright clothing that could attract insects.

People with a history of anaphylaxis should wear a medical identification bracelet or necklace to alert others in the event of another reaction. In addition, ask your doctor if you should carry a pre-loaded syringe of epinephrine (adrenaline), a medicine used to treat anaphylaxis. At the first sign of symptoms, you or a helper (family member, co-worker, school nurse) can inject the pre-loaded epinephrine to help control your allergic reaction until you reach medical attention.

Treating allergies

Although the best way to treat allergies is to minimize exposure to the allergens, many of them, such as pollen, are impossible to avoid completely. There are many medicines, both prescription and non-prescription, to treat allergic symptoms from airborne allergens and pet dander. Non-prescription antihistamines and decongestant pills can be used to relieve nasal congestion and itchy eyes. Decongestant nasal sprays can give relief for a few days, but they can cause congestion to get worse if they are used for more than three days.

Older antihistamines such as diphenhydramine (Benadryl, many generic versions) can make you drowsy. They can help if your allergy symptoms keep you up at night. During the day, people more commonly use long-acting non-sedating antihistamines, such as fexofenadine (Allegra) and loratadine (Claritin). You may need to work with your doctor to find the specific antihistamine that best relieves your symptoms with the fewest side effects.

Corticosteroid nasal sprays are probably the most effective treatment for both seasonal and year-round allergic rhinitis. Examples include beclomethasone (QNSAL, Beconase), budesonide (Rhinocort), and fluticasone (Flonase). People with seasonal allergies should start the nasal corticosteroid spray a week or two before an expected rise in pollen counts.

Montelukast (Singulair) can be used alone or in combination with other medications to treat allergic rhinitis. It is a leukotriene receptor antagonist, which decreases the allergic response in a different way than either antihistamines or corticosteroids. It is taken by mouth once a day.

Your doctor may recommend allergy shots, also called immunotherapy. This treatment is aimed at desensitizing your body's immune reaction to your specific allergens. For this treatment, increasing concentrations of the allergen are injected under your skin either weekly or biweekly. This gradually increases your body's tolerance for the allergen. If immunotherapy is effective, allergy symptoms usually subside within six months to a year. Treatment is often continued for three to five years.

Symptoms of anaphylaxis usually require an injection of epinephrine.  This is the most important treatment for anaphylaxis symptoms because it can help prevent any throat swelling from progressing to a blocked airway, which could otherwise result in suffocation. Low blood pressure may need treatment with intravenous (IV) fluids or with medication to raise blood pressure. Symptoms can also be improved with antihistamines, anti-heartburn medicines known as H2 blockers, and corticosteroids such as prednisone. It is important to see a doctor right away for allergy reactions that are severe, and for all reactions that are treated with epinephrine.

When to call a professional

Make an appointment with your doctor if you are not sure whether your symptoms are due to allergies, or if your symptoms are not controlled to your satisfaction with over-the-counter medications.

Call for emergency assistance immediately whenever you or a person you are assisting has symptoms of anaphylaxis. If you have a history of severe allergic reactions and have not mentioned this to your doctor, schedule an appointment soon. He or she can review your history and help you take the necessary precautions to avoid future problems.


In most cases, common allergy symptoms can be managed successfully by minimizing exposure to the allergens and treating with one or more medications.

With prompt, appropriate treatment, most patients who have had a severe allergic reaction, such as anaphylaxis, recover completely. A person who has had anaphylaxis is at risk of future severe reactions if he or she is exposed again to the same allergen.

Additional info

American Academy of Allergy, Asthma & Immunology (AAAAI)

Further information

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