What is an Allergy?

What is an allergy?

An allergy is the response of the body’s immune system to normally harmless substances, such as pollens, foods, and house dust mite. Whilst in most people these substances (allergens) pose no problem, in allergic individuals their immune system identifies them as a ‘threat’ and produces an inappropriate response. This can be relatively minor, such as localised itching, but in more severe cases it cause anaphylaxis, a condition which can lead to upper respiratory obstruction and collapse and can be fatal.

Allergies are very common. They’re thought to affect more than 1 in 4 people in the UK at some point in their lives. They’re particularly common in children. Some allergies go away as a child gets older, although many are lifelong.

The most common causes of allergic reactions are:

  • Pollen from trees and grasses
  • Proteins secreted from house dust mites
  • Moulds
  • Foods such as peanuts, tree nuts, milk and eggs
  • Pets such as cats and dogs, and other furry or hairy animals such as horses, rabbits and guinea pigs
  • Insects such as wasps and bees
  • Medicines (these may cause reactions by binding to proteins in the blood, which then trigger the reaction).

What happens when you have an allergic reaction?

When a person comes into contact with a particular allergen they are allergic to, a reaction occurs. This begins when the allergen (for example, pollen) enters the body, triggering an antibody response. When the allergen comes into contact with the antibodies, these cells respond by releasing certain substances, one of which is called histamine. These substances cause swelling, inflammation and itching of the surrounding tissues, which is extremely irritating and uncomfortable.

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

Common symptoms of an allergic reaction include:

  • Sneezing
  • Wheezing / coughing / shortness of breath
  • Sinus pain / runny nose
  • Nettle rash / hives
  • Swelling
  • Itchy eyes, ears, lips throat and mouth
  • Sickness, vomiting & diarrhoea

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Anaphylaxis and Severe Allergic Reactions

Anaphylaxis is a life threatening severe allergic reaction. It is a medical emergency, and requires immediate treatment. A severe allergic reaction can cause an anaphylactic shock and must be treated with an Adrenaline Auto Injector (AAI).

The most common causes of anaphylactic reactions include:

  • certain foods (including peanuts, tree nuts or shellfish). However, all foods can potentially cause anaphylaxis
  • insect stings
  • drugs and contrast agents (used in some x-ray tests), particularly those given by injection.

More information on anaphylaxis

Allergies during childhood

In the UK, 40% of children have been diagnosed with an allergy. The four most common allergies in children are food allergy, eczema, asthma, and hay fever. Allergy symptoms can affect all aspects of a child’s day to day life, including their health and wellbeing, education, and social activities.  

Allergies in children can be distressing for both the child and the parent. We understand that parents are concerned about protecting their children against allergen triggers and serious allergic reactions. 

More information on allergies during childhood

Diagnosing an allergy?

The first step in managing an allergy is identifying the cause(s) of the problem. Diagnosing allergy can be difficult since the symptoms may be similar to other conditions. You may be referred by your GP to a specialist allergy service and our helpline can tell you where your nearest specialist clinic is and give you details to take to your GP.

If you think you may be allergic to something and do not know what it is, you should start to keep a record of your symptoms. In particular, the following information may help your doctor make a diagnosis:

  • Do your symptoms occur at any particular time of the day?
  • Do you only get symptoms at certain times of the year?
  • Do you suffer more at night time or during the day?
  • Do your symptoms occur when you are in the house as well as outside?
  • Does exposure to animals bring on your symptoms?
  • Do you think that any food or drink brings on your symptoms?
  • Do the symptoms occur every time you come into contact with the allergen?
  • Do your symptoms improve when you are on holiday?

Allergy testing

If you think you have an allergy, tell your GP about the symptoms you’re having, when they happen, how often they occur and if anything seems to trigger them.

Your GP can offer advice and treatment for mild allergies with a clear cause. If your allergy is more severe or it’s not obvious what you’re allergic to, you may be referred for allergy testing at a specialist allergy clinic.

If you are looking for help and advise or information, call our Helpline on 01322 619898 – they can advise on your nearest NHS allergy clinic or consultant.

Skin prick testing

Skin prick testing is one of the most common allergy tests. It involves putting a drop of liquid onto your forearm that contains a substance you may be allergic to. The skin under the drop is then gently pricked. If you’re allergic to the substance, an itchy, red bump will appear within 15 minutes.

Most people find skin prick testing not particularly painful, but it can be a little uncomfortable. It’s also very safe.

Make sure you do not take antihistamines before the test, as they can interfere with the results.

Blood tests

Blood tests may be used instead of, or alongside, skin prick tests to help diagnose common allergies. A sample of your blood is removed and analysed for specific antibodies produced by your immune system in response to an allergen.

There are two general types of allergy blood tests:
  • A total IgE test is used to measure the total amount of IgE antibodies in your blood.
  • A specific IgE test measures how much IgE your body makes in response to a single allergen. A separate test is done for each allergen that may be causing your allergies.

Patch tests

Patch tests are used to investigate a type of eczema known as contact dermatitis, which can be caused by your skin being exposed to an allergen.

A small amount of the suspected allergen is added to special metal discs, which are then taped to your skin for 48 hours and monitored for a reaction.

Elimination diet

If you have a suspected food allergy, you may be advised to avoid eating a particular food to see if your symptoms improve.

After a few weeks, you may then be asked to eat the food again to check if you have another reaction.

Do not attempt to do this yourself without discussing it with a qualified healthcare professional.

Challenge testing

In a few cases, a test called a food challenge may also be used to diagnose a food allergy. During the test, you’re given the food you think you’re allergic to in gradually increasing amounts to see how you react under close supervision.

This test is riskier than other forms of testing, as it could cause a severe reaction, but is the most accurate way to diagnose food allergies.

Challenge testing MUST always carried out in a clinic where a severe reaction can be treated if it does develop.

Allergy testing kits

The use of commercial allergy-testing kits is not recommended.

These tests are often of a lower standard than those provided by the NHS or accredited private clinics, and are generally considered to be unreliable. Allergy tests should be interpreted by a qualified professional who has detailed knowledge of your symptoms and medical history.

How do I manage my allergy?

You can find specific information on a range of allergies on our website but here are three key things to remember when it comes to managing your allergy:

  1. Documenting where and when a reaction occurs
  2. Reducing the risk of an allergic reaction by avoiding the allergen, wherever possible
  3. Medical treatments to reduce symptoms including medications and immunotherapy.

Avoidance is the best defence against allergies and medication will relieve symptoms but will not change the allergic response.  Allergies have a significant impact on peoples’ lives.  They live in fear of a severe allergic reaction, planning their lives or those of their children around safe food choices, avoiding situations where they are at risk and carefully planning the things that most of us take for granted, such as travel and social occasions, to avoid the triggers that make them ill and could cause a fatal reaction.

There are also several medicines available to help control symptoms of allergic reactions, including;

  • Antihistamines – these can be taken 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 that can be used as a short-term treatment for a blocked nose
  • Lotions and creams, such as moisturising creams (emollients) – these can reduce skin redness and itchiness
  • Steroid medicines – sprays, drops, creams, inhalers and tablets that can help reduce redness and swelling caused by an allergic reaction


Antihistamines are probably the best known type of allergy medication, and most are readily available from a pharmacy without prescription. However, there are a number of different types of antihistamines; some have been used for many years, some are improvements on old drugs, and new antihistamines are being developed all the time. Antihistamines can be taken when you notice the symptoms of a reaction, or before being exposed to an allergen, to stop a reaction occurring

While antihistamines used to have a reputation for making people drowsy, more modern antihistamines only occasionally have those side effects.

How do antihistamines work?

During an allergic reaction, the immune system releases a chemical called histamine which starts a cascade effect of allergy symptoms. The histamine itself can lead to narrowing of airways and widening of blood vessels causing swelling or oedema (where fluid leaks into the surrounding tissue) or a drop in blood pressure. The effect of histamine in the tissues is also responsible for the itching that is associated with many allergic reactions.

Antihistamines work by blocking the action of histamine. They work best when taken prior to exposure to the allergen. However, they can also be taken after an allergic reaction has started, and this is useful for blocking the release of further histamine, reducing new symptoms.

Antihistamines are very safe. Although usually taken as tablets, they may be prescribed as a liquid or syrup for young children, or in cream form, which is very popular in first aid kits in case of insect bites or stings. Nasal sprays and eye drops containing antihistamine properties are also available, and are very useful for soothing irritated noses and eyes.


For some people with very severe allergies, a treatment called immunotherapy may be recommended. Immunotherapy, often referred to as desensitisation, is a unique treatment for allergic diseases. It is a well-established treatment reserved for certain severe allergies such as when someone has had a serious allergic reaction to wasp or bee venom or severe allergic rhinitis that has not been previously controlled by any of the anti-allergy medication tried. New Immunotherapy products are being developed and used for specific food allergy such as peanut.

Immunotherapy usually involves the administration of increasing doses of allergen extracts over a period of time, given to patients by injection or drops/ tablets under the tongue (sublingual). Food allergen desensitisation aims to reduce reactivity to the allergen and is done under very controlled medical conditions, (currently only available privately) but newer approaches to administration (such as by a skin patch that is worn and replaced) may become available in the future. These types of approaches to food desensitisation are very new and not widely available yet, but it something to provide hope for in those at risk of severe food allergic reactions.


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