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Allergies are the body's response to a substance called an allergen. Allergens themselves may not be harmful. However, in some people, things that are usually harmless can provoke a reaction. Allergic reactions vary and include many different symptoms. Some reactions can be severe and threaten life - this is called anaphylaxis. Some people have a condition called atopy which makes them prone to allergies. Testing for allergies can be complicated, and is not always needed. Medication, including antihistamines, can work well for many types of allergy.



What is an allergy?

An allergy is a response by the body's immune system to something (called an allergen) that is not necessarily harmful in itself. Certain people are sensitive to this allergen and have a reaction when exposed to it. Some allergic reactions are mild and harmless, but others are severe and potentially life-threatening (anaphylaxis).

What is a food intolerance?

A food intolerance is not the same as a food allergy. Many people incorrectly use the words interchangeably. A food allergy is when the body's immune system reacts abnormally to specific foods. No allergic reaction takes place with a food intolerance. People with a food intolerance may get digestive symptoms such as diarrhoea, bloating and stomach cramps. These are quite common symptoms anyway. In food intolerance the symptoms may be caused by difficulties digesting certain substances in food. One example is lactose, a sugar found in milk and dairy products.

Differences between food allergy and intolerance include:

  • The symptoms of food intolerance occur usually a few hours after eating the food. Allergic reactions happen much more quickly.
  • With allergy, even a tiny amount of the food can cause an allergic reaction to take place. Some people with a severe allergy to nuts might suffer anaphylaxis after eating something made in a factory that also handles nuts, or after kissing someone who has recently eaten nuts. With food intolerances you need a lot more of the food to cause the symptoms.
  • Food intolerances are never life-threatening. Some allergies are - they can cause anaphylaxis.



    How common are allergies?

    Allergies are very common. About 1 in 4 people in the UK are affected by an allergy at some time during their life. Each year the number of affected people increases.

    It is estimated that half a million people in the UK have had an anaphylactic reaction to venom (from bees or wasps). Nearly a quarter of a million people under the age of 45 have had anaphylaxis due to nuts.




    What are the symptoms of an allergy?

    Allergic reactions can vary and may include a number of different symptoms. So, with an allergy you may get one or more of the following:

    • Rhinitis (inflammation of the nose). This causes runny nose or nasal congestion and sneezing.
    • Conjunctivitis (inflammation of the eyes). This leads to watering, itching and a hot feeling in the eyes.
    • Skin rashes. The typical allergic rash is an urticarial rash - also known as hives or nettle rash. It is very itchy. Flushing of the skin is also common.
    • Angio-oedema. This is swelling of the tissues, and can include the lips, tongue, throat and eyelids. It can start with a tingling feeling. Angio-oedema is potentially very serious, as airway obstruction can occur (and so breathing might stop). People might have difficulty talking or swallowing.
    • Breathing difficulties. This includes wheezing, chest tightness and breathlessness, and can occur in severe allergic reactions and anaphylaxis. It can be life-threatening.
    • Cardiovascular collapse. This can cause death. It is the end stage of anaphylaxis. The chemicals released by the body in an extreme allergic reaction can make blood pressure drop dramatically. This can lead to loss of consciousness and to cardiac arrest (the heart stops). Resuscitation is required.
    • Other symptoms. These can include a sense of impending doom, abdominal pain, nausea and vomiting, and palpitations.



      Common allergens

      Things that people are commonly allergic to include:

      • Tree and grass pollens.
      • House dust mite.
      • Animals, especially domestic pets such as cats and dogs.
      • Insect venom such as that contained in wasp and bee stings.
      • Medicines - for example, the antibiotic penicillin.
      • Foods, such as nuts and eggs.
      • Chemicals such as latex.

      Of course, there are a great many other allergens, too many to list. Most allergens are proteins, but some (for example, medications) are not, and need to be bound to a protein once they are in the body before they can cause an allergic response.






      What happens in an allergic reaction?

      During an allergic reaction, a complex series of events occurs within the body. These events are co-ordinated by the immune system. Sometimes the immune system 'goes into overdrive'. If this happens, the body can lose control of its vital functions, with catastrophic results. Such a severe reaction can cause death.

      An allergen is regarded by the immune system as a foreign substance. When the immune system detects an allergen, it produces an immune system protein called an antibody. Antibodies are also called immunoglobulin E (IgE). The immune system stores this in its memory (this is called sensitisation). This means that you do not have an allergic reaction the first time you come into contact with a specific allergen.

      If it meets this substance again, the immune system remembers the previous exposure. Antibodies help to attack the invading allergen the immune system believes to be dangerous. A chain reaction is set up whereby other chemicals are released by different blood cells. These chemicals cause the symptoms of an allergic reaction. Histamine is one such chemical (hence, antihistamines are medications often used to counter the effects of an allergic reaction).





      Oral allergy

      Some people get a type of allergic reaction to certain foods that only causes symptoms in the mouth and throat. It tends to cause itching, tingling, swelling of the mouth, lips and throat. Fresh fruit, vegetables and nuts commonly cause this. It can be confused with anaphylaxis. It has the potential to be serious, as swelling in the mouth and throat can affect the ability to breathe, but this is rare. The symptoms start within minutes of eating and tend to settle completely within an hour. An ambulance should be called immediately if you feel faint, have difficulty breathing or feel like your throat is closing up.





      Anaphylaxis is a life-threatening allergic reaction. It is a medical emergency. If you suspect someone has anaphylaxis, you should dial 999 for an ambulance.

      The symptoms of anaphylaxis include:

      • Breathing problems and angio-oedema.
      • Collapse and loss of consciousness.




        Some families seem particularly prone to allergies. They have a condition known as atopy and are hence known as atopic individuals. People in atopic families can develop problems such as asthma, eczema and hay fever. It is an inherited problem and these people are more likely to develop an allergic disorder. Atopic individuals seem to produce more of the antibody IgE, related to allergic reactions.

        Allergy testing

        This is commonly requested, but not always useful.

        Tests can be expensive and do not always reveal a cause. Many people mistakenly believe there is a blood test they can have to check for any specific allergy.

        It is more useful to prove a suspected allergy, rather than to screen randomly for any allergy. This is because virtually anything can potentially cause an allergic reaction. There are an infinite number of allergens - certainly too many to test for. Most allergy tests involve testing for a few common allergies.
        If you suspect you have a specific and significant allergy, you should discuss this with your GP. Commercial allergy testing kits are not recommended. Do not waste money on private non-medical testing, particularly tests bought from the internet or tests that involve analysing hair or electrical impulses in the fingers. Many people have been incorrectly diagnosed with allergies and intolerances this way, and, as a result, exclude foods from their diet unnecessarily. Sometimes this can put their health at risk.

        Skin prick tests

        Skin prick allergy tests can be used to identify the cause of allergy in some circumstances. Suspected allergies to aero-allergens, especially animal danders and pollens, are particularly appropriate. Testing for some food allergies can be performed this way too.
        The allergens are mixed with liquid to make a solution. A drop of each liquid is placed on the arm and a needle is used to prick the skin, beneath the droplet of fluid. This means that a tiny amount of allergen can breach the skin. The skin is observed for allergic reaction. A positive reaction is where the skin becomes itchy and red, and there is a raised white swelling called a weal surrounding the reddish areas. This settles down over a few hours. 


        Blood tests

        Blood tests can be done in some circumstances, for some suspected allergies.
        Blood tests are safer in cases where a person has had a very extreme allergic reaction, such as anaphylaxis. Skin prick testing in these people would be potentially dangerous. Blood tests for allergies can be carried out whilst people remain on antihistamine medication (skin prick testing cannot), so it is useful if people have such severe symptoms that they cannot stop their medication.


        Patch testing

        This form of testing is used for cases of skin allergies. This is called contact dermatitis and is where people develop patches of eczema (dermatitis) as a reaction to certain substances (allergens) that the skin is in contact with. This includes certain metals, plastics and rubbers, and chemicals found in products applied to the skin.


        Treatment of allergies

        Avoidance of the cause

        Treatment with medication often works so well that you may not have much motivation or need to avoid the cause of the allergy. However, some people may wish to try to avoid the cause of the allergy, particularly if medication is not fully effective. There are a number of measures you can take to reduce the amount of common aero-allergens.


        Treatment with nose sprays, eye drops and/or antihistamine tablets will often ease or clear the symptoms. The treatment is the same as for any cause of allergic rhinitis or allergic conjunctivitis.

        Immunotherapy (desensitisation)

        This treatment is sometimes used, mainly in allergy cases that are most severe (such as reactions causing anaphylaxis) and where symptoms are not helped much by other treatments.

        People with life-threatening reactions to wasp or bee stings, those with severe hay fever and some people with some animal allergies are suitable for immunotherapy. Immunotherapy is usually unhelpful if you have multiple allergies, food allergies or allergic rashes.

        It is done using a series of injections to desensitise the immune system. The allergen you are allergic to is administered in tiny quantities, via an injection. The amount used is too small to provoke the allergic reaction, but enough to teach the immune system not to produce the huge quantities of IgE antibody. Increasing doses of allergen are given at regular intervals (usually weeks-months).

        Immunotherapy is time-consuming, expensive and carries a degree of risk. For this reason it needs to be carefully supervised by a specialist, and performed in a hospital outpatient setting. It would only be tried if other methods had previously failed.

        A newer technique involves placing the allergen under the tongue (drops or tablets). This type of immunotherapy is increasingly being used in allergy clinics, but is still not widely available. Grass pollen allergy is being treated in this way in some areas. Often, long-term treatment (up to three years) is needed for this sublingual (under the tongue) method.