An allergy is when your body’s immune system reacts to normally harmless substances that it sees as harmful. The allergy-causing substances are called allergens.


Your body’s immune system is a natural defence against infection and other foreign material. If you are allergic to something, your immune system will react every time you come into contact with that allergen.

Causes of allergies

Many substances can cause an allergic reaction, but some of the most common are:

  • pollen
  • mould
  • animal hair
  • dust and dust mites
  • latex
  • medicines
  • insect stings
  • foods.

Children and allergies

Allergies are common in childhood and may begin with the development of food allergies in children under 12 months old. Milk (dairy), eggs and peanuts are the most common food allergies in this age group, but fish, shellfish, tree-nuts, sesame, kiwifruit, wheat and soy can also be a problem.

Many children grow out of their food allergies in late childhood, although allergies to peanuts, tree-nuts, fish and shellfish tend to remain.

Eczema, which affects up to a third of children under 12 months of age, is related to the development of food allergies but may also be triggered by non-food allergens (things that trigger allergies) such as house dust mites. Most children with eczema do not have food allergies, but most children who have food allergies have eczema.

Allergies often run in families, but not every family member may be allergic to the same thing. Children living in homes with smokers are more likely to develop asthma.

Kinds of allergic reaction

Different allergens will cause different kinds of allergic reactions. The most common allergic conditions are hay fever, asthma and skin problems (eg, eczema, rashes, hives).

  • Airborne allergens such as mould, dust, pollen, grasses and weeds can cause hay fever.
  • Pollens, moulds and house dust can trigger asthma attacks.
  • Skin reactions can be caused by contact with an allergen (like latex or certain metals), by insect bites or stings, or by eating food or taking medicine you are allergic to.


The symptoms of an allergic reaction depend on the type and severity of the reaction.

Common symptoms include:

  • sneezing
  • watery and/or swollen eyes
  • stuffy or runny nose
  • itching
  • a rash or hives (raised red itchy areas on the skin)
  • stomach cramps
  • diarrhoea.

A fast pulse and nausea and/or vomiting are less common reactions.

When to call an ambulance

Sometimes an allergic reaction may be severe. This is called anaphylaxis. Anaphylaxis is a life-threatening emergency, which can affect breathing and circulation within minutes.

Call 111 straight away if you or a family member has an allergic reaction with any of these symptoms:

  • trouble breathing, including wheezing
  • swelling of lips or tongue
  • pale, cool, damp skin
  • drowsiness, confusion or loss of consciousness.

Insect stings, drugs such as penicillin, and certain foods are some of the more common causes of severe allergic reactions.

When to see your doctor

See your doctor if you or a family member has allergy symptoms that are causing you problems or you are worried that the allergy might be severe.

Your doctor will ask about your history of symptoms and examine you. To identify a food allergy your doctor may suggest that you avoid the food for a while, then try eating it again to see if your symptoms return.

Your doctor may do a skin scratch or prick test, which will check for reactions to tiny amounts of possible allergens placed under your skin. Blood tests can also help identify what you are allergic to.

Call Healthline 0800 611 116 if you are unsure what you should do.


For mild symptoms

Mild symptoms may not need treatment. A variety of over-the-counter pharmacy medicines can help with allergy symptoms, including decongestants and antihistamines.

For more severe symptoms

For more severe symptoms, your doctor may prescribe steroid medication or quick-acting inhaled bronchodilators and/or other medicine to treat breathing problems.

Immunotherapy is another possible treatment designed to make you less sensitive to an allergen so you don’t react to it so strongly. This involves injecting you with tiny but increasing amounts of the allergen. The treatment phase can last up to 6 months, followed by a maintenance phase for 2–3 years or longer.

Epi-pen emergency kits may be prescribed by your doctor for severe reactions. The epi-pen is a ready-to-use syringe of adrenaline, which is essentially designed to keep you alive until you get to hospital. Teach family members, friends and work colleagues about how to use an epi-pen and make sure they know that you must also get to hospital straight away.


If you have an allergy, you should avoid the thing you are allergic to, like by not eating or touching it.

Airborne allergens like pollen and dust mites can be especially hard to avoid. Go to Asthma and allergies for advice.

Cigarette smoke can make hay fever and asthma worse. Not smoking and not being around smokers can help.

Preventing allergies from developing

There is no known way to prevent allergies – however, some research has shown that breastfed babies may be less likely to develop allergies and asthma. Children who live in smokefree homes are also less likely to develop asthma.

Back to top