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
An allergen is any substance that causes your body's immune system to overreact and produce antibodies against it.
There are thousands of allergens, but some of the most common include:
- house dust mites
- grass and tree pollens
- pet hair or skin flakes
- fungal or mould spores
- food (particularly milk, eggs, wheat, soya, seafood, fruit and nuts)
- wasp and bee stings
- certain medication, such as penicillin and aspirin
- household chemicals
Who is at risk
Some people are more likely to develop an allergy because it runs in their family. If this is the case, you 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 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 a number of factors seem to increase the chance of a child developing atopy, such as:
- growing up in a house with smokers
- exposure to 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.
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 will also want to know if any family members have similar symptoms, or if there is a family history of allergy.
After asking about your allergy history, your GP may carry out tests to identify the allergen that is causing your symptoms, or refer you to a hospital clinic.
Even if you think you know what is causing the allergic reaction, you may need to be tested to determine the exact allergen and get a definite diagnosis. Each allergen is tested individually based on suspected or likely allergens. It is not always possible to identify the allergen causing the problem.
The type of test you are offered will depend on your symptoms, the condition of your skin and any medication you are taking. Possible tests include:
- Skin prick test. This is usually the first test to be done when looking for an allergen. The skin is pricked with a tiny amount of the suspected allergen to see if there is a reaction. If there is, the skin around the prick will very quickly become itchy, red and swollen. Because the skin prick test introduces such a tiny amount of allergen into the skin, the testing is considered very safe and can be used on almost any age group, including babies. However, it may not be suitable if you have a history of severe reaction to an allergen.
- Blood test. This is used to measure the amount of IgE antibodies in your blood that have been produced by your immune system in response to a suspected allergen. The results are given on a scale from zero to six: zero indicates a negative result and six indicates an extremely high sensitivity. Blood tests are particularly useful when you are at risk of an extreme reaction or when a rare allergen is suspected.
- Patch test. This test is used to find an allergen causing eczema (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.
The use of commercial allergy testing kits is not recommended. These tests are often of a lower standard than those used in the health services. Also, allergy tests should be interpreted by a qualified professional who has detailed knowledge of your symptoms and medical history. Some allergy testing services sold to the public have no scientific basis and cannot be relied upon.
Wherever possible, the most effective way of managing an allergy is to avoid all contact with the allergen causing the reaction (see Allergies - Prevention for more information).
Medication cannot cure your allergy, but can be used to treat the common symptoms of an allergy, such as a runny nose, itchy mouth and sneezing. Most treatments are available over the counter but always ask your pharmacist or GP for advice before starting to take any new medication.
Antihistamines and topical steroides
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.
Steroid sprays are often used to treat nasal allergy.
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.
Decongestant sprays or drops should only be used for 5 to 7 days.
Leukotriene receptor antagonists
Leukotriene receptor antagonists are tablets that block the effects of leukotrienes, chemicals released during an allergic reaction that cause inflammation (swelling) of your airways. They are used to treat asthma when other treatments have failed.
Another form of treatment for allergies is hyposensitisation, also known as immunotherapy.
Hyposensitisation works by gradually introducing more and more of the allergen into your body to make it less sensitive to it.
The allergens are usually given as injections under the skin of your upper arm. In the initial stages of treatment you will be given injections at intervals of a week or less, while allergen doses are gradually increased. When you reach the "maintenance dose", a dose that is effective in reducing your normal allergic reaction, you will need to continue to have injections of this dose every few weeks for at least two years.
Hyposensitisation is normally only recommended for the treatment of severe allergies (such as hay fever and pet allergies) that have not responded to other treatments, and for specific allergies such as bee and wasp stings.
This type of treatment must only be carried out under the close supervision of a doctor because there is a risk that it may cause a serious allergic reaction.
Anaphylaxis is a severe form of allergy. If you have an anaphylactic shock, you will need emergency treatment with an injection of a medicine called adrenaline. If you suspect that you or someone you know is having anaphylactic shock dial 999 and ask for an ambulance. If you have an allergy that could cause anaphylactic shock, or if you have had a severe allergic reaction in the past, you may be given an auto-injection kit of adrenaline. This 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.
See the Health A-Z topic for more information on the treatment of anaphylactic shock.
The best way to prevent an allergic reaction is to avoid the allergen that causes it. This is not always easy. Allergens such as 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 to avoid the most common allergens.
House dust mites
One of the biggest causes of allergies are dust mites. Dust mites are microscopic insects that breed in household dust. Below are a number of ways that you can limit the amount of mites in your house.
- Choose wood or hard vinyl floor coverings instead of a carpet.
- Fit roller blinds that can be easily wiped clean.
- Clean cushions, soft toys, curtains and upholstered furniture regularly, either by washing or vacuuming.
- Use synthetic pillows and acrylic duvets instead of woollen blankets or feather bedding.
- Use a vacuum cleaner fitted with a HEPA (high efficiency particulate air) filter, because it can remove more dust than ordinary vacuum cleaners.
- Wipe surfaces with a damp, clean cloth, as dry dusting can spread the allergens further.
Concentrate your efforts at controlling dust mites in the areas of your home where you spend the most time, such as the bedroom and living room.
It is not the pet fur that causes an allergic reaction, it is exposure to flakes of their dead skin, saliva and dried urine.
If you cannot permanently remove a pet from the house, you may find the following tips useful.
- Keep pets outside as much as possible, or limit them to one room, preferably one without carpet.
- Do not allow pets in bedrooms.
- Wash pets at least one a fortnight.
- Groom dogs regularly outside.
- Wash all bedding and soft furnishings on which a pet has lain.
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.
Moulds can grow on any decaying matter, both inside and outside the house. The moulds themselves are not 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 someone dries wet clothes next to a fireplace.
Some ways that you can prevent mould spores are outlined below.
- Keep your home dry and well ventilated.
- When showering or cooking, keep internal doors closed to prevent damp air from spreading through the house and use extractor fans.
- Do not dry clothes indoors, store clothes in damp cupboards, or pack clothes too tightly in wardrobes.
- Deal with any damp and condensation in your home.
By law, food manufacturers must clearly label any foods that contain something that is known to cause an allergy, such as celery, cereals, crustaceans, eggs, fish, milk, mustard, nuts, sesame seeds, soybeans and the preservatives sulphur dioxide and sulphites. By carefully checking the label for the list of ingredients, you should be able to avoid an allergic reaction.
Some people experience an allergic reaction while eating out at a restaurant. If you have food allergies 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, more commonly known as hay fever, are caused when plants release pollen particles into the air (pollinate). 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 and summer.
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 most pollen in the air.
- Shower, wash your hair and change your clothes after being outside.
- Avoid grassy areas, such as parks and fields.
- Get someone else to cut the grass for you if you have a lawn.
If you have ever had a severe allergic reaction (anaphylaxis), you should carry two EpiPens or Anapens with you everywhere you go.
Wear a MedicAlert or Medi-Tag medallion or bracelet, so people are aware of your allergy in an emergency, and consider telling your teachers, work colleagues and friends so they can give you your adrenaline injection in an emergency and call an ambulance. Following this advice could save your life.