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 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).
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 occurs 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 have 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 experience 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 lives. 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.
A manifesto from the European Academy of Allergy and Clinical Immunology (EAACI) published in 2017 is calling for concerted policy action to tackle the 'allergy crisis in Europe'. It points out that allergy is the most common chronic disease in Europe. Up to 1 in 5 patients with allergies live with a severe debilitating form of their condition; they struggle daily with the fear of a possible asthma attack, anaphylactic shock, or even death from an allergic reaction. The manifesto proposes a series of evidence-based recommendations to tackle the burden of allergy in Europe, foster allergy research and help strengthen allergology as a medical speciality. It is available in 'Further Reading and References', below.
Who develops allergies?
Anyone can have an allergy. About half of people with allergy are children. Some people are more prone to allergic problems due to a condition called atopy (see below). Food is a common trigger in children whilst, in older people, medicines are common culprits.
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 and sneezing.
- 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 or nettle rash. It is very itchy. Flushing of the skin is also common.
- A swelling of the tissues (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 and breathlessness, and can occur in severe allergic reactions and anaphylaxis. This 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 the heart stopping (cardiac arrest). Resuscitation is required.
- 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).
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; however, some (for example, medications) are not. These 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 immunoglobulins. An immunoglobulin commonly involved is called 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).
Do different allergies cause different symptoms
Substances which cause an immediate release of allergy chemicals such as histamine into the bloodstream cause generalised symptoms (for example, generalised itching of the body). However, some substances cause a local reaction, depending on which part of the body they first come into contact with.
Some people, for example, have a type of allergic reaction to certain foods that only causes symptoms in the mouth and throat. This involves itching, tingling, and 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.
Another example is pollen, which may cause localised symptoms such as stuffy nose, itchy eyes and wheezing.
Anaphylaxis is a life-threatening allergic reaction. It is a medical emergency. If you suspect someone has anaphylaxis, you should dial 999/112/911 for an ambulance.
The symptoms of anaphylaxis include:
- Breathing problems and a swelling of the tissues (angio-oedema).
- Collapse and loss of consciousness.
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.
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 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 the front of your forearm.
It involves putting a drop of liquid on to your forearm that contains a substance to which you you may be allergic. The skin under the drop is then gently pricked with a lancet. The site is inspected after about 15 minutes and compared with positive and negative controls, to detect sensitisation to allergens. If you're allergic to the substance, an itchy, red bump will appear within 15 minutes.
Most people find skin prick testing not particularly painful, but it can be a little uncomfortable. It's also very safe.
Serum-specific IgE testing
These blood tests to help identify any particular allergen that is causing your allergy symptoms are widely available. They are often used alongside or instead of skin prick testing. However the results are not immediate and 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, they must be interpreted in the context of your clinical history. Some people have positive test results but do not develop symptoms of allergy symptoms when exposed to the relevant allergen (false positive). Therefore randomly testing for many different food allergens is not recommended as there is a high false positive rate. 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 there being an allergy to that particular allergen, 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.
If there has been a previous severe reaction to a known food, a repeat challenge is not usually arranged for at least two years.
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. This includes certain metals, plastics and rubbers, and chemicals found in products applied to the skin.
Allergic reaction treatment
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 nasal 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.
This treatment is sometimes used, mainly when allergy symptoms are severe and have not been helped much by other treatments.
It is done using a series of injections to desensitise the immune system. 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. The amount used is too small to provoke an allergic reaction, but is enough to gradually stop the immune system from producing the huge quantities of IgE antibody that cause the allergy symptoms.
Immunotherapy is time-consuming and expensive, and it carries a small risk of a severe reaction. Therefore the injections can only be performed in a specialist clinic under the supervision of a doctor. However, drops or tablets can usually be taken at home.
Immunotherapy will not necessarily cure your allergy, but it will make it milder and easier to deal with.
Further reading and references
Food allergy in children and young people; NICE Clinical Guideline (February 2011)
Drug allergy: diagnosis and management of drug allergy in adults children and young people; NICE Clinical guideline (September 2014 updated November 2018).
Anaphylaxis; NICE Clinical Guideline (December 2011 - last updated August 2020)
Food allergy; NICE CKS, October 2018 (UK access only).
Simon D; Recent Advances in Clinical Allergy and Immunology 2019. Int Arch Allergy Immunol. 2019180(4):291-305. doi: 10.1159/000504364. Epub 2019 Nov 6.
Griffiths RLM, El-Shanawany T, Jolles SRA, et al; Comparison of the Performance of Skin Prick, ImmunoCAP, and ISAC Tests in the Diagnosis of Patients with Allergy. Int Arch Allergy Immunol. 2017172(4):215-223. doi: 10.1159/000464326. Epub 2017 Apr 29.