An allergy is an exaggerated immune system reaction to a substance that is harmless to most people. These substances are called allergens, and common examples include pollen, dust mites, mold, pet hair and dander, certain foods, insect stings and some medications [1].
In an allergy, the problem is not that the allergen is actually dangerous, but that the immune system mistakenly identifies it as a threat. The body then triggers a defensive response that can cause sneezing, itching, a runny nose, watery eyes, rash, swelling, coughing, difficulty breathing or digestive symptoms [1].
The immune system is a complex protective system whose role is to recognize and eliminate real threats such as bacteria, viruses, parasites and other harmful substances. Under normal circumstances, it distinguishes between what is dangerous and what is harmless [1].
In allergies, this assessment does not work properly. A substance such as pollen or peanut can be treated as a serious threat even though it is not harmful to most people [1].
In many allergies, an antibody called immunoglobulin E, or IgE, plays an important role. When a person first becomes sensitized to a specific allergen, the immune system begins producing IgE antibodies directed against that allergen [2].
These IgE antibodies then bind to the surface of cells called mast cells and basophils. Mast cells are found in the skin, airways and the lining of the nose, eyes and digestive system, where the body often comes into contact with the external environment [2].
When the person encounters the same allergen again, the allergen binds to IgE antibodies on mast cells. This activates the mast cells and leads to the release of histamine and other inflammatory substances [2].
Histamine is one of the main reasons typical allergy symptoms occur, including itching, sneezing, swelling, redness, a runny nose and watery eyes [3].
Allergy symptoms depend on where the reaction takes place. If the allergic reaction occurs in the nose, symptoms may include sneezing, nasal congestion, a runny nose and itching. If the eyes are affected, redness, watering and itching are common. If the skin is involved, rash, hives, redness or swelling may occur [1].
Food allergy can cause itching in the mouth, swelling of the lips or tongue, nausea, vomiting, abdominal pain or diarrhea. In some people, an allergic reaction can be severe and affect the airways or circulation [4].
The most serious form of allergic reaction is anaphylaxis. It may include difficulty breathing, swelling of the throat or tongue, a sudden drop in blood pressure, a rapid pulse, dizziness, confusion or loss of consciousness. Anaphylaxis is a medical emergency and requires immediate medical attention [4].
Many immediate allergic reactions are mediated by IgE antibodies, but not all allergies are IgE-mediated. Milk allergy, for example, is not the same as lactose intolerance: allergy involves the immune system, while intolerance involves difficulty digesting lactose because of absent or reduced activity of the lactase enzyme [4].
Similarly, a person may develop skin irritation from a product without having a true allergic reaction. This is why it is important to distinguish allergy, intolerance and irritation, because prevention and treatment approaches differ [1].
The exact reason why someone develops an allergy is not always clear. Genetic predisposition, a family tendency toward allergic diseases and various environmental factors may play a role [1].
People with a family history of allergies, asthma or eczema have a higher risk of developing allergic conditions themselves. This tendency is called atopy [1].
Allergies can appear in childhood, but they can also develop later in adulthood. Symptoms can also change over a lifetime. One person may have mild seasonal symptoms for years, while another may develop stronger reactions or become sensitive to additional allergens [1].
The most common inhaled allergens include pollen from grasses, trees and weeds, dust mites, mold and pet allergens. They most often cause symptoms of allergic rhinitis, such as sneezing, a runny nose, nasal congestion and itching of the nose and eyes [5].
Common food allergens include milk, eggs, peanuts, tree nuts, fish, crustacean shellfish, wheat, soy and sesame. Food allergies can cause mild symptoms, but in some people they can also lead to serious reactions [4].
Allergic reactions can also be triggered by medications, latex, bee and wasp stings, as well as certain ingredients in cosmetics or hygiene products. If a medication allergy is suspected, conclusions should not be made without professional evaluation; the reaction should be reviewed with a physician [1].
Sensitization means that the immune system has already recognized a specific allergen and produced specific antibodies against it. A person may be sensitized to an allergen, but this does not always mean that clear symptoms will occur with every exposure [2].
This is why allergy test results should not be interpreted in isolation. Results should be considered together with actual symptoms, the timing of symptoms and the circumstances in which the reaction occurs [4].
Allergies are usually identified through a combination of symptoms, personal and family history, examination and, when needed, allergy testing. Tests may include skin tests or blood tests for specific IgE antibodies [4].
Testing should be targeted. Random testing for a large number of allergens can produce confusing results, especially when a person has no clear symptoms associated with a specific allergen [4].
The first step is to track symptoms. It can be useful to record when symptoms occur, how long they last and whether they are associated with a season, food, time spent in a certain environment, contact with animals, cosmetics or medications [1].
If symptoms are mild and seasonal, a pharmacist or physician may recommend appropriate measures such as allergen avoidance, saline nasal rinsing, antihistamines or other medications for symptom control [5].
If difficulty breathing, swelling of the tongue or throat, a feeling of choking, weakness, fainting or a sudden worsening of symptoms occurs, immediate medical attention is required [4].
An allergy is an immune system reaction to substances that are harmless to most people. In many allergies, IgE antibodies, mast cells and histamine play key roles, which helps explain symptoms such as itching, sneezing, a runny nose, swelling and rash [2].
The most important distinction is between a true allergy, an intolerance and simple irritation. When symptoms are mild, identifying triggers and using appropriate treatment can significantly improve quality of life. When symptoms are severe, especially if they affect breathing or involve throat swelling or a drop in blood pressure, immediate medical attention is necessary [4].
[1] American Academy of Allergy, Asthma & Immunology (AAAAI). Allergies Symptoms, Diagnosis, Management & Treatment. Accessed 2026.
https://www.aaaai.org/conditions-treatments/allergies
[2] National Center for Biotechnology Information, National Institutes of Health. Janeway's Immunobiology: Effector Mechanisms in Allergic Reactions. Accessed 2026.
https://www.ncbi.nlm.nih.gov/books/NBK27112/
[3] MedlinePlus, National Library of Medicine. Histamine: The Stuff Allergies Are Made Of. Accessed 2026.
https://medlineplus.gov/medlineplus-videos/histamine-the-stuff-allergies-are-made-of/
[4] MedlinePlus, National Library of Medicine. Food Allergy. Accessed 2026.
https://medlineplus.gov/foodallergy.html
[5] American Academy of Allergy, Asthma & Immunology (AAAAI). Hay Fever | Rhinitis Symptoms, Diagnosis, Management & Treatment. Accessed 2026.
https://www.aaaai.org/conditions-treatments/allergies/hay-fever-rhinitis
Health Disclaimer: This content is for educational and informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional before starting or changing any supplement, especially if you are pregnant, breastfeeding, have a medical condition, or take medication.
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