What are Allergies?

The term allergy encompasses a wide range of conditions; it is not a disease in itself. An allergy occurs when the body’s immune system becomes hyperreactive to a substance that could be harmless in itself, called an allergen.

The external substances that provoke allergies are called allergens. The exposure may occur when the allergen is inhaled, swallowed, injected, or comes in contact with the eyes, airways, or skin. The immune response is not because of the noxious nature of the allergen but because of a misdirected recognition of the substance as harmful.

Image Credit: Created with the assistance of DALL·E 3Image Credit: Created with the assistance of DALL·E 3

The immune system is part of the body and comprises cells and their chemicals, which work together to keep the body free of harmful agents such as infections. In certain people, this reaction is triggered by pollen, some foods, certain fungi called mold, dust mites, and insect stings.

Any substance which is recognized by the immune system is an antigen. In the case of people with allergies, the allergens are picked up by certain cells, called antigen-presenting cells, which process them and allow them to be recognized by and to alert the innate immune system to their presence.

This causes the activation of any patrolling lymphocytes (a type of immune cell) that come into contact with these cell-presented antigens. In non-allergic people, when a B-lymphocyte recognizes that it has just found a foreign antigen, it returns to the lymph node nearest to its current location and transforms into a plasma cell.

Plasma cells are antibody factories. They are engineered to produce vast amounts of specific IgE built to lock on to precisely that antigen that was encountered by the B-lymphocyte. Thus, the activated B lymphocyte eventually pours out a flood of specific IgE antibodies that attach themselves to any circulating mast cells or basophils they meet.

When this complex meets an immune cell that presents the same antigen, it locks on to it in a three-way death grip while the basophil or mast cell bursts open its packages of inflammatory mediators like histamine.

The result is the classic symptoms of allergy. These include a blocked or runny nose, violent sneezing, breathing problems due to airway swelling, inflammation of the sinuses, gastritis and nausea, skin rashes, and itching.

The problem is not with the immune response itself, but that the allergic individual’s immune system cannot distinguish between harmful and non-harmful foreign proteins. This is why such people react violently to truly harmless antigens.

The curious thing about many allergies is that the first exposure does not cause any reaction. What does happen is that the person becomes sensitized – the immune system takes note of the foreign molecule or allergen and begins to mount its hostile response, all ready for the subsequent encounter.

Some of the plasma cells formed from the first B-lymphocyte that reacted to the antigen turn into memory cells that preserve the long-term memory of the antigen. The next time the person comes across this allergen in even a minor exposure, an allergic reaction occurs in the form of rapid large-scale specific antibody production against the antigen encountered.

In some cases, the allergy takes a life-threatening form, with the airways swelling so much as to practically prevent airflow, the blood vessels dilate so much that the blood pressure drops steeply, and the person may vomit or faint.

Most allergies produce symptoms first on the skin, the airways and the mucous membranes, and the intestines because of the release of histamine from mast cells in these regions. While symptoms typically begin immediately, it may take hours or days in some individuals before the sensitization occurs.

Allergies are also called hypersensitivities, but all hypersensitivities are not allergies. For instance, milk intolerance is caused by the absence or deficiency of an enzyme called lactase that breaks down milk sugar (lactose).

As a result, this sugar accumulates in the intestine, providing a breeding ground for various bacteria, which causes the typical symptoms of abdominal cramping, flatulence, and diarrhea due to the toxic byproducts of excessive bacterial activity.

History of allergies

In 1906, Clemens von Pirquet was the first to describe allergies as a changed or altered immune system reaction in response to exposure to foreign proteins. Nowadays, the term is taken to denote an exaggerated reaction to foreign substances.

How common are allergies?

Allergies are quite common. About a quarter of people in the United Kingdom have had an allergy at some time in their lives, and many of these are children.

This number is on the rise due to increasingly high levels of pollution, but also perhaps due to obsessive cleanliness that denies the developing immune system a chance to exercise its teeth on really noxious antigens.

As a result, it begins to turn on every strange substance it sees, whether neutral, friend, or foe. This is called the hygiene hypothesis and has found quite a few takers over the past decades.

Another possible contributor to the rise of allergies in the general population is the presence of atopy. Atopy is the hereditary tendency to develop allergic reactions like allergic rhinitis, asthma, and atopic dermatitis or eczema in individuals who hyper-react to inhaled or food allergens.

Allergies and their complications are expensive. According to the Asthma and Allergic Foundation of America (2002), allergies are the sixth most significant cause of chronic disease in the United States. Yearly allergies cost an estimated $18 Billion.

Types of allergen

Allergens that may be life-threatening in allergic individuals are usually harmless in the non-allergic. Common allergens include:

  • mold spores growing in damp spaces
  • house dust mites
  • grass and tree pollen
  • pet hair or skin flakes
  • food allergens, mainly from nuts, fish and shellfish, soy, and eggs

Food Allergy

Image Credit: TassaneeT/Shutterstock.com

Skin allergens or contact allergens include:

  • latex
  • nickel
  • rubber
  • preservatives
  • dyes like hair dyes

Other allergens include bee or wasp stings, drugs like sulfonamides and penicillin antibiotics, aspirin and prostaglandin inhibitors, and some anesthetics.

Diagnosis of allergies

While it is easy to diagnose the presence of an allergy, finding the cause is more complicated. It is usually achieved using a skin prick test or allergen test, where minute amounts of multiple potential allergens are injected into the skin, often on the back or arm.

An allergic swelling around any prick could signify an allergy to that substance. Patch testing for slower allergies, blood tests, and provocation tests are also carried out in situations where they are required.

Treatment of allergies

The best way to manage an allergy is to prevent it by avoiding possible or proven allergens. For instance, cleaning the house to get rid of dust mites and fungal spores, keeping the windows closed in pollen season, and dehumidifying the air to prevent mold accumulation can help prevent house allergies.

If an allergy strikes, antihistamines and steroids are used to suppress the symptoms of the attack, but they cannot cure the allergy itself.

The Anatomy of Allergies


  • InformedHealth.org [Internet] 2017. Allergies: an overview. https://www.ncbi.nlm.nih.gov/books/NBK447112/
  • Aaaai.org. (2020). Allergies. https://www.aaaai.org/conditions-and-treatments/allergies

Further Reading

Article Revisions

  • Oct 24 2023 - Improvement to general tone of article and punctuation. Main image replaced with larger image.

Last Updated: Oct 24, 2023

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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