AN INTRODUCTION TO ALLERGIES
Allergic Hypersensitivity simply put,is an adverse exaggerated immune reaction to a protein(or allergen)in our environment and which is normally harmless to the non-allergic person.An allergy may be present as a mild itching of the skin,tissue swelling and wheezing.Allergy develops after repeated exposure to the causative allergen and you don’t usually react on first exposure. Common Allergic Manifestations include Asthma,Allergic Rhinitis or Hay fever, Atopic Eczema,Contact Dermatitis,Food Allergy, Allergic Conjunctivitis and allergy to medication such as Penicillin and Anaesthetics.
Allergy testing involves having a skin or blood test to find out what substance,or allergen,may trigger an allergic response in a person. Skin tests are usually done because they are rapid,reliable and generally less expensive than blood tests,but either type of test may be used.
– Skin Tests
A small amount of a suspected allergen is placed on or below the skin to see if a reaction develops.There are three types of skin tests:
i. Skin prick test:-This test is done by placing a drop of a solution containing a possible allergen on the skin and a series of scratches or needle pricks allows the solution to enter the skin.If the skin develops a red,raised itchy area it usually means that the person is allergic to that allergen.This is called a positive reaction.
ii.Intradermal test:-During this test,a small amount of the allergen solution is injected into the skin.An intradermal allergy test may be done when a substance does not cause a reaction in the skin prick test but is still suspected as an allergen for that person.The intradermal test is more sensitive than the skin prick test but is more often positive in people who do not have symptoms to that allergen.
The health professional doing the skin prick or intradermal test will:
First clean the test site(usually on your back or arm) and then place drops of the allergens on your skin.He will then prick the skin under the drop with a needle.The needle passes through the drop and allows some of the allergen to penetrate your skin.For the intradermal test,a needle is used to inject the allergen solution deeper into the skin. He will thereafter check the skin(12 to 15 minutes later)for red,raised itchy areas.If such an area forms,it means you are possibly allergic to that allergen(this is called a positive reaction).
iii.Skin patch test:-For a skin patch test,the allergen solution is placed on a pad that is taped to the skin for 24 to 72 hours.This test is used to detect a skin allergy called contact dermatitis.
Skin testing helps in:-
a.Identify inhaled(airborne)allergens,such as tree,shrub and weed pollens,moulds,dust,feathers and pet dander.
b.Identify likely food allergens(such as eggs,milk,peanuts, nuts,fish,soy,wheat or shellfish).
c.Find out whether a person may have a drug allergy or be allergic to insect venom.
Allergy blood tests look for substances in the blood called antibodies.Blood tests are not as sensitive as skin tests but are often used for people who are not able to have skin tests.The most common type of blood test used is the enzyme-linked immunosorbent assay(ELISA).It measures the blood level of a type of antibody(called immunoglobulin E, or IgE)that the body may make in response to certain allergens.IgE levels are often higher in people who have allergies or asthma.Other lab testing methods, such as radioallergosorbent testing(RAST)or an immunoassay capture test(ImmunoCAP,UniCAP or Pharmacia CAP),may be used to provide more information.
The health professional drawing your blood will:
a.Wrap an elastic band around your upper arm to stop the flow of blood.This makes the veins below the band larger so it is easier to put a needle into the vein.
b.Clean the needle site.
c.Put the needle into the vein and attach a tube to the needle to fill it with blood.
d.Apply a gauze pad or cotton ball over the needle site as the needle is removed.
The blood sample will be placed on specially treated paper and sent to a lab to determine whether antibodies to any of the allergens being tested are present.If specific antibodies are found, it may mean you are allergic to a certain allergen.Results of allergy blood tests are usually available in about 7 days. The levels of immunoglobulin E(IgE)antibodies for a particular allergen or group of allergens are above the normal level in an affected individual.
Skin tests for allergies are the easiest and least expensive method for identifying allergies in most people. They are more reliable than blood tests for identifying common inhaled(airborne) allergies,such as pollens and dust.It however cannot tell whether a person is reacting to a certain food.This test can be unpleasant for children and their parents.
Blood tests for allergies are less sensitive than skin tests.So it is possible for people to have a positive reaction to a skin test but have a negative reaction to a blood test.They are not affected by antihistamines,so people do not need to stop taking these medicines while being tested.It is a good choice if you have very sensitive skin or a skin problem,such as eczema,that would make it hard to see whether you have a reaction to a skin test.
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