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.
Allergens are microscopic protein substances that are common and provoke allergic people to produce antidotes (antibodies).
The most common allergy provoking substances are:
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.