What is an allergy?

April 2017

The word allergy is Greek. It means abnormal, excessive and a different reaction. Indeed, people with allergies do not react like others, they "overreact" when their body comes into contact with foreign substances.

An allergic reaction is an abnormal and excessive response of the immune system to usually inoffensive substances that can be breathed in, swallowed, or touched. Normally, our immune system makes sure the body defends itself against such things and also bacteria or viruses.

An allergic reaction follows contact with a foreign substance, known as allergens, which are wrongly considered by our cells as dangerous. So, a completely inoffensive substance such as pollen or a type of food can cause an allergic reaction in some people.

On the first exposure to an allergen, whether is was breathed in, skin contact, or ingested, our body manufactures specific antibodies directed against the allergen, know as IgE (Immunoglobulin E). This sensitising phase has no symptoms.

It is at the second exposure to the same allergen that a reaction occurs., The antibodies previously made react by causing a chain of inflammatory allergic reactions which can cause rhinitis (runny nose), asthma, red, itchy swollen eyes or a skin reaction such as a rash.

Generally, allergic symptoms tend to occur in genetically predisposed people. It is thought that 50% of the population suffers with allergy, with at least one member in each family having allergic reactions.

List of the most common allergens, substances considered to be foreign by the body


Thousands of allergens are know to cause allergic symptoms. Among the most common ones are;
  • Dust mites
  • Pollens
  • Animal dander
  • Insects
  • Moulds
  • Latex
  • Cockroaches
  • Food
  • Medication, vaccines, anaesthetics
  • House plants
  • Costume jewellery
  • Pollution


Tobacco is not an allergen, except in exceptional situations, for some people working in tobacco factories. It is a nonspecific irritating substance which worsens allergic reactions.


According to where the allergic reaction occurs, the allergy will cause:

  • Allergic rhinitis, if it occurs in the nose.
  • Allergic conjunctivitis if it occurs in the eyes.
  • Eczema or hives if it occurs on the skin.
  • Angioedema if it occurs on the lips or throat.
  • Anaphylactic shock: it is the most severe allergic reaction; it can impact on all the organs in the body and be fatal if left un-treated.

Each allergen can cause one or more symptoms in an isolated or associated way

  • So, an allergy to dust mites, pollens and animals can cause rhinitis and/or asthma and/or eczema and/or conjunctivitis, but more rarely hives, angioedema or anaphylactic shock.
  • A food allergy is more likely to cause hives, but it can also cause nasal or breathing related symptoms such as rhinitis or asthma, and more rarely eye symptoms such as conjunctivitis.

An allergen, such as dust mites or pollen, can cause one or more of these symptoms, separately or along side other symptoms. Some think that pollens for example cause only allergic rhinitis and that food allergy manifests itself only in the skin (hives, eczema). This is not the case, and can therefore delay a diagnosis: pollens can for example also cause hives and food cause sneezing.

Related


Allergie - Qu'est ce que c'est ?
Allergie - Qu'est ce que c'est ?
¿Qué es la alergia?
¿Qué es la alergia?
Latest update on April 2, 2013 at 07:47 PM by Crashounette.
This document, titled "What is an allergy?," is available under the Creative Commons license. Any copy, reuse, or modification of the content should be sufficiently credited to CCM Health (health.ccm.net).