Allergies

What causes allergies?

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).

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.

See the separate leaflets called Asthma, Atopic eczema and Hay fever for more details.

Patient picks for Allergies

Allergies, blood and immune system

How to travel safely if you have allergies

Ahh holidays. Spending your time relaxing in the sun or seeing the sights in a dream destination should be blissful. But if you or your child suffer from allergies, planning a trip can be stressful. We look at how to minimise risk and maximise relaxation for allergy sufferers.

by Gillian Harvey

Allergies, blood and immune system

Antihistamines

Antihistamines are a group of medicines which act to block the effects of the chemical called histamine in the body. Either H1 or H2 histamine receptors can be blocked by medicines, but the group commonly known as antihistamines blocks the H1 receptor. They have a number of uses, but are most often used to treat allergies.

by Dr Colin Tidy, MRCGP

Allergic reaction symptoms

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

• Inflammation of the nose (rhinitis) - causes runny nose or nasal congestion, sneezing and may cause nasal polyps.

• Inflammation of the eyes (conjunctivitis) - leads to watering, itching and a hot feeling in the eyes.

• Skin rashes - the typical allergic rash is an urticarial rash, which is also known as hives. It is very itchy. Flushing of the skin is also common. There are many causes of urticaria; not all are due to an allergy.

• Swelling of the face (angio-oedema) - this 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 - these include wheezing, chest tightness, breathlessness and asthma. See here for the link to the adult leaflet about breathing difficulties, or here for the link to the leaflet about children. This can be life-threatening.

• Sudden lowering of blood pressure (hypotension). This can lead to loss of consciousness and to the heart stopping (cardiac arrest).

• Other symptoms - these can include:

  • A sense of impending doom.

  • Tummy (abdominal) pain.

  • Feeling sick (nausea).

  • Being sick (vomiting).

  • The feeling of having a 'thumping' heart (palpitations).

  • Loose or watery poo (diarrhoea).

  • Eczema

Anaphylaxis

This is a life-threatening allergic reaction caused by some of the above symptoms.

An ambulance should be called immediately if you suspect an allergic reaction and the person has any "ABC symptoms":

• A: any swelling affecting your airway. This may feel like tightening of your throat, difficult swallowing or hoarseness.

• B: difficult breathing.

• C: collapse or feeling faint or dizzy.

Look to see if the person is wearing a medical emergency bracelet or similar. Are they carrying an adrenaline (epinephrine) pen (for example, EpiPen®)? If they are, you could save their life by administering it.

See the separate leaflet called Anaphylaxis for more details.

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.

See the separate leaflets called House dust mite and pet allergy, Nut allergy and Drug allergy.

Of course, there are a great many other allergens, too many to list.

Allergies can seriously impact a person's quality of life, for many reasons. For more information about specific allergies see the individual leaflets. UK-based support and information is available from charities such as Anaphylaxis UK and Allergy UK.

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.

1. Allergen exposure. A person is exposed to the allergen.

2. Antibodies or immunoglobulins production. When the immune system detects the allergen, it produces an immune system protein called an antibody. Antibodies are also called immunoglobulins. An immunoglobulin commonly involved is called IgE.

3. Sensitisation. 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.

4. Repeat exposure. 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.

5. Body chemicals cause an allergic reaction. A chain reaction is set up whereby other chemicals are released by different blood cells. These chemicals cause the symptoms of an allergic reaction.

Do different allergies cause different symptoms

Different immune mechanisms within the body drive different allergic reactions. These can be IgE mediated, non-IgE mediated or mixed. The different mechanisms are responsible for the type of allergic reactions the person experiences, and how rapidly they occur after exposure to the allergen.

Allergic reactions may be generalised, affecting much of the body (for example, generalised itching of the body) or they may cause a local reaction. How they come into contact with the body can also affect the reaction. An example is pollen, which may cause localised symptoms such as stuffy nose, itchy eyes and wheezing.

Allergy testing

Allergy testing is only possible for allergies which are IgE-mediated. Non-IgE-mediated allergies are not able to be tested for using a specific test. For suspected non-IgE-mediated allergies a person is usually asked to follow a diet which eliminates the possible allergen from their diet and then reintroduces it to diet to see if their symptoms occur again. This may require help from a dietician. Allergies caused by an obvious allergen, or if the reaction is mild, do not usually require testing.

Allergy testing may involve initial skin prick testing or measuring specific immunoglobulin (Ig)E levels to different allergens in the blood (serum). Commercial allergy testing kits are not recommended. These tests are often of a low standard and are generally considered to be unreliable. Allergy tests should always be interpreted by a qualified professional who has detailed knowledge of your symptoms and medical history.

• Skin prick testing. Skin prick testing is one of the most common allergy tests. The test is usually performed on your forearm. It involves putting a drop of liquid on to your forearm that contains a substance to which you may be allergic. The skin under the drop is then gently pricked with a lancet. The site is inspected after about 15 minutes. If you're allergic to the substance, an itchy, red bump may appear within this time. Most people find skin prick testing not particularly painful, but it can be a little uncomfortable. It's also very safe.

See the separate leaflet called Skin prick allergy test for more details.

• Serum-specific IgE testing. These blood tests help identify a particular allergen that is causing your allergy symptoms. However it may take days or weeks to get the results. Serum-specific IgE testing may be more appropriate when skin prick testing is not possible, or when skin prick testing gives uncertain results.

Although both skin prick testing and the IgE blood tests are very useful, the results must be interpreted in the context of your clinical history. Some people have positive test results but do not develop allergy symptoms when exposed to the relevant allergen (false positive result). Some people may have a strong history of allergy symptoms to a particular allergen but have negative allergy test results (false negative result). The bigger the size of the skin prick wheal, or the higher the concentration of serum-specific IgE, the greater the risk of the result being correct, but it does not mean that symptoms will necessarily be more severe.

• Oral food challenge. If the results of allergy testing do not correspond with the clinical history, an oral food challenge may be needed to confirm a diagnosis of food allergy and identify the cause. This is the most accurate way to diagnose food allergies. It involves giving increasing quantities of the food allergen under medical supervision, starting with direct exposure of the allergen on your lips, and, if no response on your lips, then gradual, increased amounts to eat as you can tolerate. If this does not cause any symptoms then the test is negative and allergy can be excluded. An oral food challenge may cause a severe reaction, and therefore is always carried out in a clinic where a severe reaction can be treated if it does develop.

• Patch testing. This form of testing is used for cases of skin allergies. This is called contact dermatitis - a condition in which people develop patches of eczema (dermatitis) as a reaction to certain substances (allergens) that the skin is in contact with.

See the separate leaflet called Patch testing for contact dermatitis for more details.

Allergic reaction treatment

Avoidance of the cause

Depending on the allergen, avoidance of the substance which causes the allergic reaction may be possible. this is particularly important for people with anaphylaxis.

See the leaflets regarding specific allergens for more information.

Medication

Medication to improve localised allergic symptoms, such as nasal sprays or eye drops, may be helpful. Antihistamines may be given to help control local or generalised reactions.

People who have been diagnosed with anaphylaxis should be issued with adrenaline auto-injectors and taught how to use them. For more information see the leaflets anaphylaxis and adrenaline.

Desensitisation (immunotherapy)

This treatment is sometimes used, mainly when allergy symptoms are severe and have not been helped much by other treatments.

The allergen you are allergic to is administered in tiny quantities, either as an injection, or as drops or tablets under the tongue, given in increasing doses at regular intervals (weeks or months) over the course of several years. Immunotherapy is time-consuming and expensive, and it carries a small risk of a severe reaction. This treatment option can only be performed in a specialist clinic.

Immunotherapy will not necessarily cure your allergy, but the aim is to "desensitise" the immune system to the allergen, making the allergic reactions milder.

Difference between allergy and food intolerance

A food intolerance is not the same as a food allergy. Many people incorrectly use the words interchangeably. A food allergy occurs when the body's immune system reacts abnormally to specific foods. No allergic reaction takes place with a food intolerance, instead the symptoms are caused by difficulties digesting certain substances in food.

See the separate leaflet called Food allergy and intolerance for more details.