There is a world of difference between allergy and intolerance, but most people don't know what it is.

The term 'food allergy' refers to an instant response from the immune system, when it mistakenly perceives certain proteins in food as harmful and mounts an attack against them.

A food intolerance, on the other hand, is more of a digestive system response and gives rise to delayed and more chronic symptoms.

Unlike allergy and anaphylaxis, an intolerance, such as lactose intolerance which is common, is rarely life-threatening.

Food intolerances are more common and less severe than allergies. Most people will have experienced discomfort with some types of food, a feeling sometimes described as, 'it doesn't quite agree with me'.

According to consultant paediatrician and allergy expert DrLee Bee Wah, allergies and intolerances can develop at any age, but are more commonly seen in young children.

A food allergy can be diagnosed with a pin prick test or a more sophisticated blood test called the Rast (radioallergeosorbent) test. This measures food-specific antibodies in a blood sample.

An intolerance is diagnosed through the trial-and-error method or by maintaining a food diary and identifying the food that causes discomfort later.

It can also be identified through an elimination diet, by which the person avoids a certain food like wheat or dairy to see if his condition improves.

The symptoms of the two differ greatly as well.

Symptoms of food allergy
Anaphylaxis (a life-threatening reaction)
Itchy skin
Shortness of breath
Chest pain
Swelling of the airways to the lungs
Rash or hives
Nausea
Stomach pain
Diarrhoea

Symptoms of food intolerance
Heartburn
Diarrhoea
Headaches
Nausea
Stomach pain
Gas, craps and bloating
Vomiting

This story was first published in The Straits Times' Mind Your Body supplement on Jan 30, 2008.