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Overview - Allergies

An allergy is a reaction the body has to a substance.

Allergies are very common. They affect more than 1 in 4 people in Europe at some point in their lives.

They're particularly common in children. Some allergies go away as a child gets older. Many are lifelong.

Adults can develop allergies to things they were not allergic to before.

Having an allergy can be a nuisance and affect your everyday activities. Most allergic reactions are mild and can be kept under control.

Severe reactions can occur, but these are rare.

Common allergies

Substances that cause allergic reactions are called allergens.

The more common allergens include:

  • grass and tree pollen – an allergy to these is known as hay fever (allergic rhinitis)
  • dust mites
  • flakes of skin or hair from animals
  • food – particularly nuts, fruit, shellfish, eggs and cows' milk
  • insect bites and stings
  • medicines – including ibuprofen, aspirin and certain antibiotics
  • latex – used to make some gloves and condoms
  • mould – these can release small particles into the air that you can breathe in
  • household chemicals – including those in detergents and hair dyes

Most of these allergens are harmless to people who are not allergic to them.

Symptoms of an allergic reaction

Allergic reactions usually happen within a few minutes of exposure to an allergen.

They can cause:

  • sneezing
  • a runny or blocked nose
  • red, itchy, watery eyes
  • wheezing and coughing
  • a red, itchy rash
  • worsening of asthma or eczema symptoms

Most allergic reactions are mild. Occasionally a severe reaction called anaphylaxis or anaphylactic shock can happen. This is a medical emergency and needs urgent treatment.

Getting help for allergies

Contact your GP if you think you or your child might have had an allergic reaction to something.

The symptoms of an allergic reaction can also be caused by other conditions.

Your GP can help find out if you have an allergy. If you have a mild allergy, they can offer advice and treatment to help manage the condition. They may refer you to an allergy specialist for testing and treatment, if your allergy is severe or the cause is not known.

Managing an allergy

The most effective way of managing an allergy is to avoid the allergen that causes the reaction.

For example, if you have a food allergy, check a food's ingredients list for the allergen before eating it.

Medicines that help control symptoms of allergic reactions include:

  • antihistamines – you can take these when you notice the symptoms of a reaction, or before being exposed to an allergen, to stop a reaction occurring
  • decongestants – tablets, capsules, nasal sprays or liquids used as a short-term treatment for a blocked nose
  • lotions and creams, such as moisturising creams (emollients) can reduce skin redness and itchiness
  • steroid medicines – can help reduce redness and swelling caused by an allergic reaction

If you have a very severe allergy, you may need immunotherapy. Immunotherapy exposes you to the allergen in a controlled way over a period of time. Your body gets used to it and does not react to it so severely.

Causes of allergies

Allergies happen when your body's immune system reacts to a particular substance as though it's harmful.

The number of people with allergies increases every year. The reasons for this are not understood. It could be the result of living in a cleaner, germ-free environment. This reduces the number of germs our immune system has to deal with.

Allergies, sensitivities, intolerances


A reaction the body has to a substance.


The exaggeration of the normal effects of a substance. For example, the caffeine in a cup of coffee may cause extreme symptoms, such as palpitations and trembling.


Where a substance causes unpleasant symptoms, such as diarrhoea, but does not involve the immune system. People with an intolerance to certain foods can often eat a small amount without having any problems.

Content supplied by the NHS and adapted for Ireland by the HSE

Page last reviewed: 25 March 2021
Next review due: 25 March 2024

This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 123.