What is an Allergy?

On this page you will find information about common causes of allergic reactions and their symptoms.

What is an allergy?

An allergy is the response of the body’s immune system to normally harmless substances, such as pollens, foods, and house dust mite. Whilst in most people these substances (allergens) pose no problem, in allergic individuals their immune system identifies them as a ‘threat’ and produces an inappropriate response. This can be relatively minor, such as localised itching, but in more severe cases it cause anaphylaxis, a condition which can lead to upper respiratory obstruction and collapse and can be fatal.

The most common causes of allergic reactions are:

  • Pollen from trees and grasses
  • Proteins secreted from house dust mites
  • Moulds
  • Foods such as peanuts, tree nuts, milk and eggs
  • Pets such as cats and dogs, and other furry or hairy animals such as horses, rabbits and guinea pigs
  • Insects such as wasps and bees
  • Medicines (these may cause reactions by binding to proteins in the blood, which then trigger the reaction).

Did you know...

  • 150m

    Europeans suffer from chronic allergic diseases

    (M. L. Levy, 2004)

  • 615%

    increase in the rate of hospital admissions for anaphylaxis in the UK between 1992-2012

    (Turner, Paul J., et al, 2015)

  • 44%

    of British adults now suffer from at least one allergy

    (Mintel, 2010)

What happens when you have an allergic reaction?

When a person comes into contact with a particular allergen they are allergic to, a reaction occurs. This begins when the allergen (for example, pollen) enters the body, triggering an antibody response. When the allergen comes into contact with the antibodies, these cells respond by releasing certain substances, one of which is called histamine. These substances cause swelling, inflammation and itching of the surrounding tissues, which is extremely irritating and uncomfortable.

Common symptoms associated with allergic conditions include:

  • Sneezing
  • Wheezing / coughing / shortness of breath
  • Sinus pain / runny nose
  • Nettle rash / hives
  • Swelling
  • Itchy eyes, ears, lips throat and mouth
  • Sickness, vomiting & diarrhoea

How to help your Doctor diagnose an allergy?

The first step in managing an allergy is identifying the cause(s) of the problem. Diagnosing allergy can be difficult since the symptoms may be similar to other conditions. You may be referred by your GP to a specialist allergy service and our helpline can tell you where your nearest specialist clinic is and give you details to take to your GP.

If you think you may be allergic to something and do not know what it is, you should start to keep a record of your symptoms. In particular, the following information may help your doctor make a diagnosis:

  • Do your symptoms occur at any particular time of the day?
  • Do you only get symptoms at certain times of the year?
  • Do you suffer more at night time or during the day?
  • Do your symptoms occur when you are in the house as well as outside?
  • Does exposure to animals bring on your symptoms?
  • Do you think that any food or drink brings on your symptoms?
  • Do the symptoms occur every time you come into contact with the allergen?
  • Do your symptoms improve when you are on holiday?

Allergy is the most common chronic disease in Europe. Up to 20% of patients with allergies struggle daily with the fear of a possible asthma attack, anaphylactic shock, or even death from an allergic reaction, EAACI, 2016

How do I manage my allergy?

You can find specific information on a range of allergies on our website but here are 3 key things to remember when it comes to managing your allergy:

  1. Documenting where and when a reaction occurs
  2. Reducing the risk of an allergic reaction by avoiding the allergen, wherever possible
  3. Medical treatments to reduce symptoms including medications and immunotherapy.

Avoidance is the best defence against allergies and medication will relieve symptoms but will not change the allergic response.  Allergies have a significant impact on peoples’ lives.  They live in fear of a severe allergic reaction, planning their lives or those of their children around safe food choices, avoiding situations where they are at risk and carefully planning the things that most of us take for granted, such as travel and social occasions, to avoid the triggers that make them ill and could cause a fatal reaction.


Antihistamines are probably the best known type of allergy medication, and most are readily available from a pharmacy without prescription. However, there are a number of different types of antihistamines; some have been used for many years, some are improvements on old drugs, and new antihistamines are being developed all the time.

While antihistamines used to have a reputation for making people drowsy, more modern antihistamines only occasionally have those side effects.

How they work

During an allergic reaction, the immune system releases a chemical called histamine which starts a cascade effect of allergy symptoms. The histamine itself can lead to narrowing of airways and widening of blood vessels causing swelling or oedema (where fluid leaks into the surrounding tissue) or a drop in blood pressure. The effect of histamine in the tissues is also responsible for the itching that is associated with many allergic reactions.

Antihistamines work by blocking the action of histamine. They work best when taken prior to exposure to the allergen. However, they can also be taken after an allergic reaction has started, and this is useful for blocking the release of further histamine, reducing new symptoms.

Antihistamines are very safe. Although usually taken as tablets, they may be prescribed as a liquid or syrup for young children, or in cream form, which is very popular in first aid kits in case of insect bites or stings. Nasal sprays and eye drops containing antihistamine properties are also available, and are very useful for soothing irritated noses and eyes.

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