Allergy or hypersensitivity (type 1 allergy) is an abnormal reaction to protein substances that occur naturally. If an allergic person is exposed to these substances called allergens, the body’s immune system gets ready to fight them.White blood cells (B-lymphocytes) produce an antidote (antibody) against the allergen. The antibody sticks to the surface of the allergy cells. Now the body is ready to fight back the next time it is exposed to the allergen. This process is called sensitisation.

After this change, there is an allergic reaction every time the body is exposed to the allergen.The allergen sticks to the antibodies on the surface of the allergy cells. This coupling causes the granula (little stores in the allergy cells) to release histamine, which causes the symptoms of allergy.Depending on the size of the exposure to the allergen and where on the body it happens, there will be an allergic reaction in the form of hay fever, asthma or nettle rash.

The histamine dilates the blood vessels, causes the mucous membranes (lining tissues of the nose and airways) to swell due to the liquid leaking and stimulates the glands in the nose and the respiratory passages to produce mucus (phlegm).

Substances that make the musculature of the respiratory passages contract are released along with the histamine. It becomes difficult to breathe and an asthma attack may follow.

An allergy is very different to an intolerance or sensitivity; there are on the whole no scientific replicable tests for food sensitivities, although it is now possible to test for gluten intolerance (as opposed to gluten allergy or celiac disease) with an NHS blood test available through GP’s.

What are allergens?

Allergens are microscopic protein substances that are common and provoke allergic people to produce antidotes (antibodies).

The most common allergy provoking substances are:

  • pollen from weeds, grass, flowers and trees
  • mould and mould fungus
  • house dust mites
  • fur from cats and dogs
  • medicines

Treatment of allergy

Allergy is a vast subject and some specific types and their treatments are covered in separate factsheets on hay fever and rhinitis, asthma, eczema, pet allergies and food allergies.

Minor allergies can often be treated by over-the-counter remedies, such as antihistamines, examples of which include cetirizine (eg Zirtek allergy tablets) or loratadine (eg Clarityn allergy tablets), which reduce the allergic reaction.

Other remedies are listed below.

  • Nasal products: sodium cromoglicate (eg Rynacrom nasal spray), antihistamines, such as levocabastine (eg Livostin nasal spray), decongestants and corticosteroids, such as beclometasone (eg Beconase nasal spray) – alone or in combination with other products.
  • Eye drops: sodium cromoglicate (eg Opticrom eye drops), nedocromil sodium (eg Rapitil eye drops), antihistamines, such as levocabastine (eg Livostin direct eye drops) – alone or in combination with other products. Corticosteroids are used only in severe cases.
  • Injections of small amounts of the allergy-inducing agent to create tolerance (hyposensitisation) against known allergy-inducing substances. This can prevent the immune system from producing too much histamine. Such treatment has to take place over a long time and requires strict adherence.
  • Rarely, and only in severe cases, intramuscular injections of long-acting steroids can be given, which act for weeks or months to relieve severe symptoms. However, these injections can carry a significant risk of severe side-effects and so are not now given routinely.