What is it?

Allergies (also known as ‘hypersensitivities’) are overreactions of the immune system to substances that do not cause reactions in most people. Hypersensitivities are grouped into four types - I, II, III and IV. These classifications are based on which parts of the immune system are activated and how long it takes for a reaction to occur.

The two types of hypersensitivities commonly associated with the term allergies are type I or ‘immediate’ hypersensitivities and type IV or ‘delayed’ hypersensitivities.

In type I hypersensitivity, a particular foreign substance (known as an allergen) reacts with a specific type of antibody called immunoglobulin E (IgE) – this reaction causes symptoms to appear within minutes. In type IV hypersensitivity, the allergen reacts with a specialised type of cell called a T-lymphocyte. In this case symptoms take hours to days to appear.

Type I

Type I hypersensitivities affect mainly the respiratory system (nose, throat, lungs) , the gastrointestinal system (stomach, intestines, bowel) and the skin. They occur most frequently in those with a family history of allergies (although not always to the same substance). The first time a predisposed person is exposed to a potential allergen, they will not have a major reaction; instead, they will create a specific IgE antibody and become 'sensitised'. This exposure may occur through the skin in children with atopic dermatitis (eczema); thus, a reaction might occur on the first consumption of food as is common in peanut allergy.

If that person is exposed to the allergen again, the specific IgE identifies the allergen, attaches to it, and triggers the release of chemicals, including histamine, that cause allergic symptoms. These symptoms start wherever the allergen was introduced (for example, in the mouth, nose or on the skin).

On the skin, an acute type I allergic reaction can cause a rash, dermatitis and itching, while in the long term the allergy may cause atopic dermatitis and eczema. In the respiratory tract, the acute allergic reaction causes coughing, nasal congestion, sneezing, throat tightness and in the long term, asthma. It can also cause red itchy eyes. Acute allergic reactions in the gastrointestinal system start in the mouth with tingling, itching, a metallic taste and swelling of the tongue and throat, followed by abdominal pain, muscle spasms, vomiting and diarrhoea, over time leading to a variety of gastrointestinal problems.

Any severe acute allergic reaction also has the potential to be life threatening, causing anaphylaxis, a reaction spread throughout the entire body that can start with agitation, a 'feeling of impending doom' pale skin (due to low blood pressure), shortness of breath and/or loss of consciousness (fainting). Anaphylaxis can be fatal without the rapid administration of an adrenaline injection.

Type I allergic reactions can be variable in severity, one time causing a rash, the next time anaphylaxis. Type I allergies can be to just about anything: foods, plants (pollens, weeds, grasses, etc), insect venoms (for example wasp or bee stings), animal dander (from the fur of cats and dogs), dust mites, mould spores, occupational substances (for example latex) and drugs (such as penicillin). There can also be cross-reactions, where someone allergic to grass pollen, for instance, may also react to melons and tomatoes. The most common food-related causes of severe anaphylactic reactions are peanuts, tree nuts (such as cashews) and crustaceans (such as prawns). Many other foods including the common food allergens such as eggs, milk, soy, wheat, fish and sesame may lead to anaphylaxis.

Type IV

Type IV delayed hypersensitivity reactions are most often skin reactions. Common examples include reactions to metal and jewellery. They occur when an allergen interacts with specific T lymphocytes. No immune system sensitisation is necessary; a person can have a type IV reaction with the first exposure. Type IV hypersensitivity is usually a reaction (redness, swelling, hardening of the skin, rash, dermatitis) that occurs at the exposure site hours to days after exposure.

Non-allergies

There are other reactions that can cause allergy-like symptoms but are not caused by an activation of the immune system. They range from toxic reactions that affect anyone who has sufficient exposure, such as food poisoning caused by bacterial toxins, to genetic conditions, such as intolerances caused by the lack of an enzyme (for example, the inability to digest milk sugar, resulting in lactose intolerance) and sensitivities to things like gluten (in coeliac disease). Symptoms can also be caused by medications such as aspirin and ampicillin, pain/itch receptor triggering by some food dyes and additives, food dyes, MSG (monosodium glutamate – a food flavour additive) and by some psychological triggers. While these diseases and conditions may need to be investigated by your physician, they are not allergies and will not be identified during allergy testing.

Tests and treatment

The diagnosis of an allergy starts with a careful review of the person’s symptoms, family history and personal history, including the age of onset, seasonal symptoms and those that appear after exposure to animals, hay, or dust, or that develop in specific environments (e.g. home and work). Other environmental and life style factors such as pollutants, smoking, exercise, alcohol, drugs and stress may make the symptoms worse and should be taken into consideration. Once the list of possible allergens has been narrowed, specific testing can be done.

• Allergen specific IgE testing (formally known as RAST Testing), is a laboratory test that is used to screen for allergen-specific IgE antibodies. It is often recommended when skin prick testing is not possible or when a severe allergic reaction might be anticipated. Allergen-specific IgE antibody testing involves taking a blood sample and checking for each allergen suspected. Allergens may be selected one at a time or by choosing panels such as food panels, which contain the most common adult or child food allergens, and regional weed and grass panels, which contain the most common airborne allergens in the location where the person lives. Individual selections are very specific, for example: paper wasp versus honey bee, or fish or prawn, or egg white versus egg yolk. Your doctor will help you select the most appropriate allergens. Usually someone will only be truly allergic to a few substances (four or less). If a specific IgE test is negative, the chances are that you are not allergic to that substance, but a positive test must be evaluated alongside your clinical symptoms. You can have a low specific IgE level and still have a severe reaction to actual exposure to the allergen or an elevated level and never experience a reaction. People who outgrow a food allergy may continue to have positive IgE test result to the food for many years.
• Total IgE testing is sometimes done to look for an ongoing allergic process. It is a blood test that detects the presence of IgE protein (including allergy antibodies) but does not identify specific allergens. Conditions other than allergies can also cause total IgE to rise.
  Oral food challenges are considered the 'gold standard' for diagnosing food allergies. They require close medical supervision because reactions can be severe (life threatening anaphylaxis). Food challenges involve giving you small amounts of unmarked potential food allergens and watching for allergic reactions. Negatives are confirmed with larger meal-sized portions of food.
• Elimination is another way to test for food allergies: eliminating all suspected foods from the diet, then reintroducing them one at a time to find out which one(s) are causing the problem.
• Patch testing. Delayed hypersensitivity patch tests are the easiest methods of testing for 'delayed' allergies (for example allergies to rubber or nickel). A concentration of the suspected allergen is applied to the skin under a non-absorbent adhesive patch and left for 48 hours. If burning or itching develops more rapidly, the patch is removed. A positive test consists of redness with some hardening and swelling of the skin, and, sometimes, vesicle (blister-like) formation. Some reactions will not appear until after the patches are removed, so the test sites are also checked at 72 and 96 hours.

Other tests not widely accepted as useful by the Australasian Society of Clinical Immunology and Allergy include:

• VEGA testing (the measurement of disordered electromagnetic currents in the body)
• Provocation-neutralisation ('Miller technique')
• Hair analysis
• Applied kinesiology
• Auriculo-cardiac pulse tests
• Leucocytotoxic tests (‘Nutron’ and ‘ALCAT’ tests)
• IgG4 or IgG food panel testing.