Patient Resources

What is an allergy?

The term 'allergy' is open to wide interpretation, with many people saying that they are allergic to something if it makes them feel unwell in any way. The clinical definition of an allergy is hypersensitivity (abnormal sensitivity) to a foreign substance (allergen) that is normally harmless. An allergic reaction is the body's way of trying to get rid of an allergen.

Some of the more common allergies include the following: hay fever; allergic asthma; eczema in infants; drug allergies; and food allergies. The number of people affected by allergies varies in different parts of the world. For example, in China asthma affects far fewer people than in Australia and New Zealand, while in Britain the number of people affected falls somewhere in between. These differences are thought to be the result of hereditary factors.

An allergy is different to an intolerance. For example, a food intolerance may be caused by a lack of a specific substance in the body, whereas an allergy is caused by an immune response by the body.

What are the symptoms of allergic reactions?

People respond to allergies in different ways and the symptoms vary depending on the type of allergy. Hay fever is a common allergy that causes a variety of symptoms, such as a runny or blocked nose and sneezing, and eye symptoms such as watering, redness, itchiness or a gritty feeling. It can also cause asthma-type wheeziness in the chest - most asthma-type reactions cause narrowing of the airways which makes breathing more difficult.

Another fairly common allergen is metal such as nickel. Nickel is often found in jewellery and watch straps and can cause an eczema-like rash usually referred to as contact dermatitis.

True food allergies, as opposed to food intolerances, are relatively rare. Reactions range from mild (eg, colic and diarrhoea) to life-threatening (swelling of the lips, tongue and throat, and severe asthma). Coeliac disease is a particular condition caused by an allergy to gluten, a constituent of wheat. Some food additives such as tartrazine can cause skin rashes (urticaria) or asthma.

An allergic substance may cause a serious, immediate reaction known as anaphylaxis. This can happen in response to an injection, for example, or in rare cases, to certain foods such as nuts, or to bee or wasp stings. The symptoms of anaphylaxis occur very suddenly and can include the following: generalised swelling; swelling of the throat causing difficulty in breathing; asthma symptoms; itchy rash; and faintness and unconsciousness due to low blood pressure. Anaphylaxis can be life-threatening and needs immediate treatment.

What are the causes of allergic reactions?

The incidence of allergies is thought to be increasing as a result of the healthier lifestyle we now have compared with that of our predecessors. Because many harmful viruses and bacteria have been eradicated it is thought that our immune systems are reacting increasingly to other foreign substances. The causes of allergic reactions are widespread. Potential allergens include certain foods, dust, pollens, animal fur, metal, drugs or injections, wasp or bee stings and bites from other types of insects.

The tendency to allergy can be inherited and people who are allergic to one substance are likely to have multiple allergies. For example, children with asthma have a greater tendency to suffer from eczema than those who do not have asthma. Allergies may also be related in part to the environment. For example, the pollen count in one country may be higher than in another, resulting in higher levels of hay fever in that country.

Are any tests necessary?

There are various tests that may be used to establish the cause of allergies, including skin patch testing and skin prick testing.

Skin patch testing can be used to test for skin allergies such as urticaria or dermatitis. Small discs impregnated with various allergens are applied to the back or upper arm and left in place for 48 hours. The skin will be checked for any redness or swelling immediately after removing the patch and again 48 hours later. If there is a reaction the allergen responsible can be identified from the patch that caused it.

Skin-prick testing involves placing a small amount of different allergen solutions on the skin. The skin is then pricked to allow the solutions to enter under the skin. If there is an allergic reaction to that substance, an itchy weal will appear soon afterwards. A number of allergens will usually be tested at the same time. The test is usually done on the forearm but sometimes on the back in infants. Once the allergen is known, the sufferer can minimise exposure to it where possible.

Testing for food allergies can be a long process and usually requires exclusion of certain foods or restricted diets. Patients need to be monitored carefully by a doctor or dietitian. However, because the reaction times to various foods can vary from a few hours to days it may be difficult to identify the cause of the allergic reaction.

Tests for asthma include the use of a peak flow meter and a vitalograph machine to record lung function. These may be particularly useful if a person has an allergy to a substance at work, such as grain or flour or high levels of dust. The asthma may not be present while at a doctor's surgery but, if present while at work, may indicate an allergy to a particular substance in the workplace.

What treatment is available?

Antihistamines are used to treat allergic reactions. These work by blocking the action of histamine, a substance that is produced by the body in response to an allergen and which causes the allergic reaction. Antihistamines are particularly useful in the treatment of hay fever symptoms. They are also given to relieve itching and irritation of the skin (urticaria) and for mild acute allergic reactions. They are of no benefit in asthma.

Sedating antihistamines include alimemazine, chlorphenamine (eg, Piriton®), clemastine (Tavegil®), cyproheptadine (Periactin®), hydroxyzine (eg, Atarax®, Ucerax®), ketotifen (Zaditen®) and promethazine (Phenergan®). These may cause drowsiness as a side effect, which can be useful in some conditions such as itchiness of the skin at night.

Non-sedating antihistamines are also available. These include cetirizine (eg, Zirtek®), desloratadine (Neoclarityn®), fexofenadine (Telfast®), levocetirizine (Xyzal®), loratadine (eg, Clarityn Allergy®) and mizolastine (Mizollen®). Antihistamines are available as tablets and/or liquid. Some antihistamines can be purchased from a pharmacist without a prescription.

For allergic reactions causing nasal symptoms such as sneezing or runny nose, an anti-inflammatory drug such as sodium cromoglicate may be effective. Sodium cromoglicate is available as a nasal spray (Rynacrom®). Some antihistamines are also available as nasal sprays, for example, azelastine (Rhinolast®).

Corticosteroid nasal sprays may also be given. These may contain beclometasone (eg, Beconase®, Nasobec®), budesonide (eg, Rhinocort Aqua®), flunisolide (Syntaris®), fluticasone (eg, Avamys®, Flixonase®), mometasone (Nasonex®) or triamcinolone (Nasacort®).

Decongestants such as pseudoephedrine (eg, Sudafed®) can be useful for short-term treatment of sinusitis or prior to flying. Combination products containing an antihistamine and a decongestant are also available.

For hay fever affecting the eyes, there are several antihistamine eye drop preparations available including azelastine (Optilast®) and emedastine (Emadine®).

In asthma, specific treatment is prescribed and includes the use of inhalers and other medication.

Anaphylactic reactions require immediate treatment, usually with a combination of antihistamines and adrenaline given in injection form.

Further information available from:

Allergy UK
3 White Oak Square
London Road
Swanley
Kent BR8 7AG
Helpline: 01322 619898
Email: info@allergyuk.org
Internet: www.allergyuk.org

Fact sheet provided by MIMS

Date last reviewed: March 2008

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