Allergy versus intolerance – what’s the difference?

January 2013

Dr Huissoon provides diagnosis and management of allergic and immunodeficiency disorders at Spire Parkway Hospital, Solihull.

Allergies and intolerances to foods or other substances may both cause unpleasant symptoms.  They are often confused and many people are unsure what the difference is.

Why have different names?

It is very important to distinguish the two types of reaction, because the diagnosis and treatment are different for each.  If it is not clear whether the reaction is due to allergy or intolerance, unnecessary tests may be performed or the wrong treatment given.

An allergic reaction occurs where the immune system over-reacts to a normally harmless substance (such as pollen or peanut). When suffering from intolerance, the immune system is not involved. Normally harmless substances cause symptoms, but often the mechanism is not understood.

Allergies are common and easy to identify

Most people who suffer from allergic reactions know what they are reacting to. This is usually from their own experience. They may, for example, develop an itchy rash shortly after eating nuts, on every occasion that they eat them. Or they may sneeze when in contact with cats. Allergy tests can help to confirm that they are sensitised to peanuts or cats, which, in combination with their reactions, confirms allergy as the cause of their symptoms.

What are examples of some intolerances?

Lactose intolerance: Lactose is a complex sugar that is present in milk and some other foods and drugs. An enzyme in the gut (called lactase) breaks it down into simple sugars that can be absorbed into the bloodstream. If this enzyme is missing then the lactose cannot be absorbed and stays in the gut. This causes symptoms such as diarrhoea and bloating.  Infants may get transient lactose intolerance after a stomach bug. This usually gets better by itself if milk is avoided for a few months. Lactose intolerance in adults may be long-lasting.

Migraines triggered by foods:  Some migraine sufferers know all too well that eating chocolate, strong cheeses, red wine or yeast extracts (among others) may set off a migraine attack.  These foods all contain chemicals called amines that can cause blood vessels to expand or contract, and this is probably the trigger for the migraine. Some people get other symptoms, such as flushing, runny nose or palpitations from the same foods. The mechanism is not known.

No one understands why some people get symptoms from these foods. It is certain, however, that the immune system is not involved. A range of other intolerances has been described, and many others probably exist but have never been formally studied. For example, some people are quite clear that their catarrh gets better when they avoid milk. This may well be true, but it is not an allergy and has never been explained.

How can testing help?

Allergy tests check if your immune system reacts to a substance in an allergic way. Common allergy triggers such as a nuts, fish, dust, cat hair etc. can be tested.  A positive allergy test means that you are “sensitised” to that substance.  If a sensitised person has typical allergy symptoms when exposed to that substance, then allergy is likely to be the cause of the symptoms. Allergy tests are only done when allergy seems likely from the patient’s symptoms. Allergy tests can be done on a blood sample or by skin prick testing (which is quicker, giving almost immediate results). They cannot be used for “screening”, since many people are sensitised to substances without getting any allergic symptoms from them.

What tests are available for intolerances?

This is where understanding the difference between allergy and intolerance is important.  Because no-one fully understands the mechanisms of most intolerance reactions, it is difficult or impossible to design a test that will diagnose them.  There is no point in testing the immune system, because it is not involved.  Blood tests would be expected to be entirely normal even during an intolerance reaction.

Usually the only way of knowing what triggers intolerance symptoms is through detective work.  If a food is suspected, then keep a food diary and try to relate the symptoms you experience to foods that preceded it.  This sounds easier than it is, because sometimes the symptoms take a day or two to emerge after eating the suspected food.

To make an appointment at Spire Parkway Hospital please call 0121 704 1451


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Advice provided by

Dr Aarnoud Huissoon, MB DCH MRCPI FRCP FRCPath PhD

Read more about Dr Huissoon, Consultant Immunologist, Spire Parkway Hospital.

To make an appointment at Spire Parkway Hospital call: 0121 704 1451

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