An allergy is an adverse reaction that the body has to a particular food or substance in the environment.

Most substances that cause allergies are not harmful and have no effect on people who are not allergic.

The allergic response

Any substance that triggers an allergic reaction is called an allergen. Some of the most common allergens include:

grass and tree pollen (hay fever)

dust mites

animal dander (tiny flakes of skin or hair)

food allergy (particularly fruits, shellfish and nuts)

An allergy develops when the body’s immune system reacts to an allergen as though it is a threat, like an infection. It produces antibodies to fight off the allergen, in a reaction called the "immune response". 

The next time a person comes into contact with the allergen, the body "remembers" the previous exposure and produces more of the antibodies. This causes the release of chemicals in the body that lead to an allergic reaction.

Symptoms of an allergy can include sneezing, wheezing, itchy eyes, skin rashes and swelling.

The nature of the symptoms depend on the allergen. For example, you may experience problems with your airways if you breathe in pollen.

Seeing your GP 

If you think you have an allergy, see your GP.

Depending on your symptoms, the condition of your skin and any medication you are taking, you may be offered further tests to identify the allergen.

How common are allergies?

Allergies are very common. According to Allergy UK, one in four people in the UK suffers from an allergy at some point in their lives. The numbers are increasing every year and up to half of those affected are children.

The reason for the rise is unclear. Some experts believe it is associated with pollution. Another theory is that allergies are caused by living in a cleaner, germ-free environment, which reduces the number of germs our immune system has to deal with. This causes it to overreact when it comes into contact with harmless substances.

Managing an allergy

In some cases, the most effective way of managing an allergy is to avoid all contact with the allergen causing the reaction.

There are also several medications available to help control the common symptoms of many allergies.

Symptoms of allergies 

The symptoms of an allergic reaction can vary, depending on which substance (allergen) you are allergic to.

If you are allergic to substances in the air – such as pollen, animal dander and dust mites – the symptoms usually include:

rhinitis – sneezing and a blocked, itchy or runny nose

conjunctivitis – itchy, red, streaming eyes

asthma – wheezing, breathlessness and a cough

If you are allergic to a certain food or medication, symptoms can include:

urticaria (hives) – a raised, itchy, red rash

swelling – usually affecting the lips, tongue, eyes and face

abdominal pain, vomiting and diarrhoea

atopic eczema – the skin becomes dry, red and cracked

You can also be allergic to substances coming into direct contact with the skin, such as perfumes, soaps, hair dyes and metal jewellery. This causes a type of eczema known as contact dermatitis.

It is important to remember that these symptoms can also be caused by other conditions, so see your GP for advice if you're not sure what's causing your symptoms.


The symptoms of an allergic reaction do not happen the first time you come into contact with an allergen, but at a later point of contact.

This is because the body’s immune system has to develop sensitivity to the allergen before you can become allergic to it. In other words, your immune system needs to recognise and memorise the allergen (for example, pet hair or pollen) and then make antibodies against it. This process is known as sensitisation.

The time taken to become sensitised to an allergen varies from days to years. Some people stop in the sensitisation phase, as they experience symptoms, but never fully develop an allergy.


In very rare cases, an allergy can lead to a severe allergic reaction, called anaphylactic shock, which can be fatal.

Most allergic reactions occur locally in a particular part of the body, such as the nose, eyes or skin. In anaphylaxis, the allergic reaction involves the whole body and usually happens within minutes of coming into contact with a particular allergen.

The symptoms of anaphylactic shock can include any or all of the following:

swelling of the throat and mouth

difficulty swallowing or speaking

difficulty breathing

a rash anywhere on the body

flushing and itching of the skin

stomach cramps, nausea and vomiting

a sudden feeling of weakness, due to a fall in blood pressure

collapsing and becoming unconscious

You can read about anaphylaxis for more information on a severe allergic reaction. If you have anaphylactic shock, you will require emergency treatment, usually with an injection of a medicine called adrenaline.

Causes of allergies 

An allergy develops when the body’s immune system reacts to an allergen as though it is harmful, like it would an infection.

It produces a type of antibody (protein that fights off viruses and infections) called immunoglobulin E (IgE), to fight off the allergen.

When the body comes into contact with the allergen again, IgE antibodies are released, causing chemicals to be produced. Together, these cause the symptoms of an allergic reaction.

One of the chemicals involved in an allergic reaction is histamine, which causes:

tightening of your muscles, including those in the walls of your airways

more mucus to be produced in your nose lining, causing local itching and burning

Common allergens

An allergen is any substance that causes your body’s immune system to overreact and produce antibodies against it.

There are thousands of allergens that can trigger allergies, but some of the most common include:

grass and tree pollens (hay fever)

dust mites

animal dander (tiny flakes of skin or hair)

food allergy (particularly fruits, shellfish and nuts)


Who is at risk?

Some people are more likely to develop an allergy because it runs in their family. Those who this applies to are said to be atopic, or to have atopy. People who are atopic are more likely to develop allergies, because their body produces more IgE antibodies than is normal.

Environmental factors also play a part in the development of allergic disorders. The exact role of the environment is unknown, but studies have shown that several factors seem to increase the chance of a child developing atopy, such as:

growing up in a house with smokers

exposure to house dust mites

exposure to pets

using antibiotics

Boys are more likely to develop atopy than girls, as are babies who have a low birth weight. The reasons for this are unclear.

Diagnosing an allergy 

If you think you have an allergy, tell your GP about the symptoms you are 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.

If you require allergy testing or specialist advice, you may be referred to an allergy clinic. If you have a severe or complex allergy, you may need to be seen at one of the larger allergy centres.

Find your local  allergy clinic.

Allergy tests

There are a variety of allergy tests available to help make an accurate diagnosis. These are described below.

Skin prick testing

Skin prick testing is mainly used to investigate allergies to:

airborne allergens

certain foods

insect venoms

certain drug allergies

The test involves putting a drop of liquid allergen onto your forearm, followed by a gentle pin prick through the drop. If you are allergic to the substance, an itchy, red bump will appear within 15 minutes.

Skin prick testing is very safe and can be performed in people of all ages. Antihistamines interfere with the results, so they need to be stopped for several days before the test.

Intradermal testing

Intradermal testing involves injecting a watered-down allergen into the top layer of skin with a very fine needle.

It is slightly more painful than the skin prick test and presents a slightly higher risk of an allergic reaction, but is still very safe.

Intradermal testing is usually used to investigate allergies to certain medications or insect venom.

Blood tests

Blood tests may be used instead of, or alongside, skin prick tests to help diagnose common allergies.

The tests used are known as specific IgE tests or radioallergosorbent tests (RASTs).

Patch tests

Patch tests are used to investigate a type of eczema known as contact dermatitis.

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. This test is usually carried out at a dermatology (skin) department in a hospital.

Challenge testing

A food challenge may be used to diagnose a food allergy if skin and blood tests have not given a clear diagnosis.

During the test, you're given the food you think you are allergic to in gradually increasing amounts, to see how you react under close supervision.

This test is riskier than other forms of testing, but is the most accurate way to diagnose food and certain medication allergies.

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  or accredited private clinics. Allergy tests should be interpreted by a qualified professional who has detailed knowledge of your symptoms and medical history.


A skin prick test will be the first test suggested 

Treating allergies 

The best treatment for an allergy depends on which allergen is causing the reaction.

In some cases, avoiding the substance is the most effective way of managing an allergy.

The main medical treatment for an allergy involves using medication to control the symptoms.

If you experience severe allergic reactions (anaphylaxis) you may be prescribed an adrenaline injector. For severe cases of rhinitis, immunotherapy may be recommended.


Most treatments are available over the counter, but always ask your pharmacist or GP for advice before starting any new medication.


Antihistamines treat allergies by blocking the action of the chemical histamine, which the body releases when it thinks it is under attack from an allergen. Antihistamines can be taken in tablet, cream or liquid form, or as eye drops or nasal sprays.

Nasal sprays can be used to reduce swelling and irritation in your nose, and eye drops will help to relieve sore, itchy eyes. Some sprays and drops are only suitable for adults, so always ask your GP or pharmacist for advice before buying treatments for yourself or your children.


Decongestants help to relieve a blocked nose, which is often caused by hay fever, a dust allergy or a pet allergy. Decongestants can be taken as tablets, capsules, nasal sprays or liquids. They should not be used long term.

Leukotriene receptor antagonists

Leukotriene receptor antagonists are tablets that block the effects of leukotrienes, which are chemicals released during an allergic reaction and cause your airways to become inflamed (swell). They are used to treat asthma when other treatments have failed, and as a supplement to steroid treatment.

Steroid sprays

Corticosteroid sprays designed to act on the nasal lining and airways are effective in suppressing inflammation, particularly nasal congestion. Absorption into the body is minimal, so there are no adverse side effects.

Immunotherapy (desensitisation) 

Immunotherapy may be an option for a small number of people with severe rhinitis who are unable to effectively control their symptoms by avoiding the allergen or using medication.

Immunotherapy is a course of vaccines that lasts for three years. The aim is to reduce the severity of the allergy and the amount of medication needed to control symptoms.

The vaccine contains the allergen and can be given as an injection in the arm or as drops or tablets under the tongue. The injection can only be performed in a specialist clinic under the supervision of a doctor, as there is a small risk of anaphylaxis. The drops or tablets can usually be taken at home.

Treating anaphylaxis

Some people with severe allergies may experience life-threatening reactions to their allergen, known as anaphylaxis.

Symptoms of anaphylaxis include difficulty breathing and a drop in blood pressure, leading to loss of consciousness.

If you have an allergy that could cause anaphylaxis, you will be given an auto-injection kit of adrenaline, along with a written treatment plan and appropriate training. 

The auto-injector is an easy-to-use syringe that you should carry with you at all times. The brands currently prescribed in the UK are the EpiPen and Anapen.

You might also want to consider wearing a medical information bracelet, or another form of identification that carries information about your condition.



Severe allergic reactions (anaphylaxis) are treated with an adrenaline injection 

Preventing allergies 

The best way to prevent an allergic reaction is to avoid the allergen that causes it.

This is not always easy. Allergens such as house dust mites or fungal spores can be hard to spot and can breed in even the cleanest house.

It can also be hard to avoid pets, particularly if they belong to friends and family, and many food allergies are triggered because people do not realise they are eating food they are allergic to.

Below is some practical advice that should help you avoid the most common allergens.

House dust mites

One of the biggest causes of allergies are dust mites, which are microscopic insects that breed in household dust. You can limit the amount of mites in your house by: 

choosing wood or hard vinyl floor coverings instead of a carpet

fitting roller blinds that can be easily wiped clean

cleaning cushions, soft toys, curtains and upholstered furniture regularly, either by washing (at a high temperature) or vacuuming

using tested allergen-proof covers on mattresses, duvets and pillow

using a vacuum cleaner fitted with a HEPA (high efficiency particulate air) filter, because it can remove more dust than ordinary vacuum cleaners

wiping surfaces with a damp, clean cloth, as dry dusting can spread the allergens further

increasing ventilation and reduce humidity by opening windows and using extractor fans or dehumidifiers

Concentrate your efforts of controlling dust mites in the areas of your home where you spend the most time, such as the bedroom and living room.


It's not the pet fur that causes an allergic reaction. Instead, it's exposure to flakes of their dead skin, saliva and dried urine.

If you can't permanently remove a pet from the house, you should try: 

keeping pets outside as much as possible, or limiting them to one room, preferably one without carpet

not allowing pets in bedrooms

washing pets at least once a fortnight

regularly grooming dogs outside

washing all bedding and soft furnishings on which a pet has lain (ideally at 60°C)

If you are visiting a friend or relative with a pet, ask them not to dust or vacuum on the day you are visiting, as this will stir up the allergens into the air. Taking an antihistamine medicine one hour before entering a pet-inhabited house can help to reduce symptoms.

Mould spores

Moulds can grow on any decaying matter, both inside and outside the house. The moulds themselves aren't allergens, but the spores they release are. Spores are released when there is a sudden rise in temperature in a moist environment, such as when central heating is turned on in a damp house, or wet clothes are dried next to a fireplace.

You can help prevent mould spores by:

keeping your home dry and well-ventilated

keeping internal doors closed when showering or cooking, to prevent damp air from spreading through the house, and using extractor fans

not drying clothes indoors, store clothes in damp cupboards or packing clothes too tightly in wardrobes

dealing with any damp and condensation in your home

When outdoors, you can help prevent contact with mould spores by avoiding:

damp buildings

buildings used to store hay or grain

woods and rotten leaves, especially in damp conditions

cut grass and compost heaps

Food allergies

By law, food manufacturers must clearly label any foods that contain something that is known to cause an allergic reaction. By carefully checking the label for the list of ingredients, you should be able to avoid an allergic reaction.

Many people experience an allergic reaction while eating out at a restaurant. You can avoid this by:

not relying on the menu description alone (remember, many sauces or dressings could contain allergens)

communicating clearly with the waiting staff and asking for their advice

avoiding places where there is a chance that different types of food could come into contact with each other, such as buffets or bakeries

Remember, simple dishes are less likely to contain "hidden" ingredients.

Pollen allergies

Pollen allergies – more commonly known as hay fever – are caused when plants (trees and grasses) release pollen particles into the air.

Different plants pollinate at different times of the year, so the months that you get hay fever will depend on what sort of pollen(s) you are allergic to. Typically, people are affected during spring (trees) and summer (grasses).

To avoid exposure to pollen you can:

check weather reports for the pollen count and stay indoors when it is high

avoid drying clothes and bedding outside when the pollen count is high

wear wraparound sunglasses to protect your eyes from pollen

keep doors and windows shut during mid-morning and early evening, when there is the most pollen in the air

shower, wash your hair and change your clothes after being outside

avoid grassy areas, such as parks and fields

if you have a lawn, get someone else to cut the grass for you 

Insect bites and stings

If you've ever suffered a bad reaction to an insect bite or sting, it's important to take precautions to minimise your risk.

For example, when you are outdoors, particularly in summer, you can cover exposed skin, wear shoes and apply insect repellent.

Severe allergies

If you have ever had a severe allergic reaction (anaphylaxis), make sure you carry two adrenaline auto-injectors with you everywhere.

Wear a MedicAlert or Medi-Tag medallion or bracelet so that people are aware of your allergy in an emergency. Consider telling your teachers, work colleagues and friends, so they can give you your adrenaline injection in an emergency, while waiting for an ambulance. Following this advice could save your life.