Defining the difference between an allergy to certain foods as opposed to an intolerance to certain foods is something even the experts don't agree on. However, there is one clear distinction which separates allergic reactions into a few different types.
The first category - sometimes called a Type 1 allergy or fixed allergy - occurs very soon after contact with the allergen and causes the immune system to react immediately with symptoms such as itching, rash, stomach pain, diarrhoea, swelling of the airways, shortness of breath and, in extreme cases, anaphylaxis.
This type of allergy to foods such as shellfish is relatively rare and can usually be diagnosed by means of skin prick tests or blood tests for the antibody IgE.
Food intolerances - sometimes called cyclical or slow response food allergies - are much more common and usually occur when the body lacks a certain enzyme necessary to properly digest a certain food. The immune system then produces raised levels of IgG antibodies in response to this and causes an array of symptoms.
The problem is that some of the symptoms of a food intolerance - nausea, stomach pain, gas, cramps, bloating, vomiting, headaches - are similar to allergic symptoms. However, the distinguishing feature is that a small amount of the food allergen will always cause the symptoms in IgE type allergies while it often takes a large amount of the specific food to cause the symptoms in what is known as an IgG type food allergy.
