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Allergies & immunity

Allergies: Causes, Symptoms, and Treatment

By The Tessan Editorial Team · July 19, 2026 · 7 min read

Also known as: Allergic reaction, Hypersensitivity, Seasonal allergies

What is an allergy?

An allergy is an abnormal, exaggerated immune system reaction to normally harmless substances, called allergens. These substances can be found in the environment, in food, or in certain medications. When someone with an allergy is exposed to one, their body triggers a disproportionate immune response that causes various symptoms. In the United States, roughly 1 in 3 adults and children have a seasonal allergy, eczema, or food allergy, according to the Asthma and Allergy Foundation of America (AAFA), making allergies one of the most common chronic diseases. AAFA puts the total at more than 106 million people who experience some form of allergy each year, including about 82 million with diagnosed seasonal allergies, 30 million with eczema, and 22 million with food allergies.

Causes and risk factors

Allergies result from a combination of genetic and environmental factors. Someone with allergic parents is more likely to develop an allergy, though the specific allergens a parent reacts to are not what gets passed down. Common allergens include pollen, dust mites, pet dander, mold spores, certain foods (peanuts, dairy, eggs, shellfish), insect stings, and medications like penicillin.

Environmental exposures matter too. Traffic-related air pollution is linked to reduced lung growth and to the development and worsening of asthma, and living or growing up in an urban area raises asthma risk. Smoking is another: children whose mothers smoked during pregnancy, or who breathe secondhand smoke, are more likely to wheeze and to develop asthma. Researchers have also put forward the hygiene hypothesis, the idea that more early-life infections and a larger family lower the risk of allergic disease, and children raised on rural farms do show less atopy and asthma than children growing up in nonrural environments.

The reaction itself builds in two stages. In a susceptible person, the body treats an allergen as harmful and makes IgE antibodies specific to it. On later encounters, those antibodies recognize the allergen and set off the immune response that produces symptoms.

Symptoms

Allergic symptoms vary depending on the type of allergy and the organ involved. The most common symptoms include:

In the most severe cases, an anaphylactic reaction can occur, which is life-threatening and involves a drop in blood pressure, facial swelling, and severe difficulty breathing.

Diagnosis

Diagnosing an allergy involves several steps. The doctor starts by gathering the patient’s history and details on when symptoms occur. Skin prick tests help identify the responsible allergens by observing the skin’s reaction to small doses of allergen extracts. Blood tests measuring specific IgE antibody levels can round out the evaluation. When a food allergy is in question, an oral food challenge may be done: the patient is fed tiny amounts of the suspected food in increasing doses under strict medical supervision, followed by a few hours of observation. Because the test can bring on a serious reaction, it belongs in a medical facility with an experienced allergist and the right medications and equipment on hand.

Available treatments

Managing allergies relies on several complementary approaches. Start with avoiding the responsible allergen: it is very important, though not always easy, and with a food allergy staying away from the food is the only way to prevent a reaction. Medication-wise, antihistamines relieve sneezing and itching in the nose and eyes, and they also reduce a runny nose. Nasal corticosteroid sprays reduce inflammation and swelling in the nose and block allergic reactions from developing, and corticosteroid creams relieve itchiness and stop rashes from spreading. An allergic asthma plan includes quick-relief medicines, and the most common of these are short-acting beta-agonists, bronchodilators that open and relax the muscles of the airways. For severe reactions, epinephrine is essential, available both as an auto-injector and as a nasal spray.

Allergen immunotherapy (desensitization) is the only long-term treatment that works on the underlying mechanisms of the allergy: it gradually exposes the body to increasing doses of the allergen to reduce sensitivity over time. The American College of Allergy, Asthma & Immunology (ACAAI) describes it as the only treatment that actually changes the immune system, and notes it may help prevent new allergies and asthma from developing. It comes as allergy shots (subcutaneous immunotherapy), the most commonly used and most effective form, and as sublingual tablets taken under the tongue. Dosing is tailored to the patient and can be slowed down if reactions occur.

Prevention and practical tips

Reducing your exposure to allergens helps limit how often and how severe flare-ups are. If dust mites are your trigger, cover mattresses and pillows in zippered allergen-barrier covers, skip wall-to-wall carpeting in favor of solid-surface flooring and washable throw rugs, and keep indoor humidity between 30 and 50 percent. During pollen season, watch the pollen counts and forecasts in your local weather report, limit outdoor activities when counts are high, and keep windows closed when you can. For food allergies, check ingredient labels carefully and learn the other names your allergen goes by. Keeping a diary of what you do, what you eat, when symptoms show up, and what seems to help makes it easier to pin down your triggers.

When should you see a doctor?

See a doctor if you get hay fever or other allergy symptoms several months out of the year, if antihistamines and other over-the-counter medications don’t control your symptoms or leave you with side effects you can’t live with, such as drowsiness, or if your allergies interfere with your day-to-day activities or drag down your quality of life. A doctor can confirm which allergens are responsible through skin or blood testing and discuss whether immunotherapy is an option for you.

Anaphylaxis is a medical emergency. Call 911 right away if a reaction involves difficulty breathing, coughing or wheezing, or swelling of the face, eyes, or tongue. If an epinephrine auto-injector or nasal epinephrine is available, use it immediately: there is no contraindication to using epinephrine in a severe allergic reaction. Get to the nearest emergency facility at the first sign of anaphylaxis, even if you have already given the epinephrine.

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