Allergies

Why do people get allergies?

One function of the body's immune system is to detect and get rid of intestinal worms. When it identifies worms in the digestive system, it reacts in two ways - constricting the gut to expel them, and producing chemicals to kill them.

Unfortunately for some people, their bodies "misread" other substances they come into contact with, and react in the same way as they would to the presence of worms, even if these substances are nowhere near the digestive tract. People who react to substances in this way are said to have an allergy to them, and the substances themselves are described as allergens.

Allergic reactions vary from mild to severe (and even fatal).

How are allergies and asthma related?

Approximately 70 to 80 percent of asthma in New Zealand is associated with allergy. When someone with allergies and asthma breathes in an allergen they're sensitive to, their lungs react in the same way as the gut would react to the presence of worms. In the first two to three minutes their airways constrict, causing symptoms such as wheezing and tightness in the chest, and within four to six hours the lungs produce chemicals which cause inflammation and further breathing difficulties.

Sometimes more than one part of the body can be affected by an allergen, which is why many people with asthma also experience hay-fever and eczema.

What are the most common allergens for people with asthma?

In New Zealand, the most common asthma-producing allergens are dust mite faeces, cats' hair and saliva, pollen, rodent urine, and mould.

Doesn't your body get used to allergens the more it is exposed to them?

No. In fact, the reverse is the case. Once you are "sensitised" to an allergen and have asthma - that is, once you start getting an asthma reaction to allergens - the more you're exposed to them, the more sensitive to them you become.

What can people do to reduce the risk of allergy-associated asthma?

People who have asthma which is triggered by allergens can try to identify the allergen, in order to avoid or minimise exposure to it.

For some, it may be possible to do this by monitoring their exposure to an identifiable trigger to see whether it aggravates their asthma.

It may also be possible to gauge the likelihood that a substance is a trigger with a diagnostic test such as a skin prick test and/or a special blood test. A skin prick test involves testing a small amount of the suspected trigger on the skin to see if a reaction occurs. A positive reaction takes the form of an itchy red swelling with a white area called a "weal", which usually develops in 15 to 20 minutes and then subsides within a couple of hours. As a positive reaction does not always mean the substance reacts in the lungs, other information such as clinical symptoms is also taken into account in determining whether to consider the substance an asthma trigger. Anyone who is interested in one of these tests, or would like more information about them, should discuss this with their doctor. Note that skin prick tests are generally not used for children under the age of three, being less reliable for very young children.

With children, remember that early exposure to allergens may be of significance in the development of asthma. Try to minimise their exposure to the common allergens, in order to minimise the risk.

For further information on house dust mites and asthma, refer to the Asthma Foundation's House Dust Mites fact sheet.

© The Asthma and Respiratory Foundation of New Zealand (Inc), 12/2001  Photocopy permission granted