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What is an Allergy?

Jessica Ellis
Updated Mar 03, 2024
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An allergy is an immune system response to a substance the body perceives as harmful. Although the substance, called an allergen, may be harmless, an abnormal immune system will respond to contact, inhalation or ingestion of the allergen as an attack. The body's response to an allergen, known as an allergic reaction, can be as minor as sneezing or as serious as potentially fatal system shock. Common substances that create an allergic reaction include antibiotics, nuts, plants, pet dander and insect venom.

Symptoms vary depending on the allergy, but many present like common cold symptoms. Low fever, sneezing, nasal congestion, and a sore or scratchy throat may all indicate allergies. Many reactions to chemical allergens, such as latex or parabens, include a bumpy or itchy skin rash called contact dermatitis. Serious reactions should be treated with prompt medical care, and include swelling, asthma attacks, or the narrowing of airways.

What causes an allergic reaction is a question still eluding science, although certain factors are generally held responsible. Many types of allergies have dramatically increased since the mid-20th century, leading some experts to suggest that increased environmental pollution has made immune systems more hypersensitive to attacks. One hypothesis suggests that the high standards of hygiene in the modern world limit a child's exposure to infection and allergens during crucial developmental phases, making a hypersensitive immune system more likely.

It remains unknown why allergies will develop in one person and not another. Some experts suggest that allergies are often genetically inherited, with identical twins being particularly likely to share the same issues. Often, an allergic reaction will develop in childhood, but since many reactions tend to worsen with repeated exposure, symptoms can start to appear at any age. Contrarily, some people are able to gradually desensitize their system with exposure, meaning that adults may overcome childhood allergies.

Testing for an allergy can be a tricky process, particularly if an allergic reaction could trigger life-threatening conditions. Doctors often employ a skin-patch method of testing for a variety of possible allergic reactions. In skin testing, a tiny dose of an allergen is injected into a marked patch of skin to see if the skin reacts negatively. If serious reactions are a concern, doctors can also use a blood sample, although this takes more time and may be less conclusive than skin testing.

Treatment may depend on the type and severity of the patient's allergic response. In some mild cases, such as sneezing caused by animal dander, patients are told to avoid exposure when possible and take an antihistamine when exposure is necessary. Avoiding the allergic trigger is also key with many food allergies, particularly those that can cause severe responses.

When avoidance is impossible, some doctors may recommend shots. This treatment doses the patient with small amounts of the allergen intended to build up tolerance in the immune system. The doses usually begin at a minute level unlikely to cause a response, and increase over time to typical exposure levels.

Allergies are becoming more and more common around the world, but most are minor annoyances. Seasonal hay fever or pet allergies are irritating but harmless most of the time, yet any developing an allergic reaction should be treated with care. If symptoms seem to worsen or include asthma or shortness of breath, some experts recommend seeking medical care at once.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Jessica Ellis
By Jessica Ellis , Writer
With a B.A. in theater from UCLA and a graduate degree in screenwriting from the American Film Institute, Jessica Ellis brings a unique perspective to her work as a writer for The Health Board. While passionate about drama and film, Jessica enjoys learning and writing about a wide range of topics, creating content that is both informative and engaging for readers.

Discussion Comments

By EliseP — On May 11, 2011

Doctors and scientist do know, for the most part, what allergies are caused by which are allergens like insect bites or stings, certain foods, pollen, dust, animal dander, pollution, mold and cigarette smoke. Symptoms of being allergic to allergens are sneezing, running nose, watery eyes, rashes, breathing difficulties, itchy throat, nasal congestion, stuffy ears, headaches, hives and sore throats.

Jessica Ellis

Jessica Ellis


With a B.A. in theater from UCLA and a graduate degree in screenwriting from the American Film Institute, Jessica Ellis...
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