Forbidden fruits, nuts, eggs...

It is not unusual for adults to develop allergies to food they previously enjoyed


Imagine developing an extreme allergy out of the blue in adulthood. All your favourite foods become potentially lethal, and you can no longer eat at friends' or in a restaurant, nor travel on a plane without taking elaborate precautions. In Britain alone, allergic diseases are increasing at a rate of one per cent of the population per year, and now affect almost 10 million Britons - up to one million are severely affected.

Many of the sufferers are children, and the life-threatening consequences of an allergy to peanuts, eggs and shellfish in young people are well documented. Less well-known is that an increasing number of adults are visiting doctors, because, without warning, they have begun to exhibit an extreme allergic reaction to food they previously enjoyed. Occupational allergies - to the latex powder used in surgical gloves, for example -are also becoming more common.

An allergy is an excessive immune reaction to substances known as allergens. In an allergic person, the immune system interprets allergens as damaging, and exposure triggers the production of immunoglobulin E (IgE) antibodies, to attack the invading material. This leads to the release of other chemicals including histamine, which produce symptoms ranging from a runny nose, wheezing and hives to the drop in blood pressure, swelling in the throat and shortness of breath that herald anaphylactic shock. "Allergies can develop at any age, although they are most common in children,'' says Dr Gideon Lack, consultant in paediatric allergy and immunology at St Mary's Hospital, London.

"It is not uncommon for somebody to develop a nut allergy in middle age. One of the reasons is that some nuts have a similar protein structure to that of birch tree pollen. A significant proportion of teenagers and young adults who are allergic to birch pollen and suffer from hayfever will go on to develop an allergic reaction to some nuts - usually hazelnuts and walnuts - and certain stone fruit, including peaches.

"Only a minority of these will have an anaphylactic reaction, but the problem with a mild food allergy is that you do not know whether it is going to become severe. And the psychological effects can be extreme, because you never know where you are going to meet the forbidden ingredient. It can make life rather frightening.''

Oral Allergy Syndrome, as this condition is known, has become more prevalent in the past 10 years, says Stephen Durham, professor of allergy and respiratory medicine at the Royal Brompton Hospital, London. "It could be to do with the general increase in allergies in the past 20 years, which has been blamed on the so-called hygiene hypothesis,'' says Prof Durham. "We live in a more sterile environment, with fitted carpets, central heating, less time spent outdoors, smaller families and therefore less cross-infection in early childhood.

"We do not know why Oral Allergy Syndrome suddenly develops. It only occurs in people with a previous history of allergic reactions. Increased exposure to the allergen may be one part of the equation: it is true that the more you are exposed to something, the more likely you are to become allergic to it.'' Some specialists believe that a trauma, such as a car accident or an illness, can trigger the onset of allergy in adulthood.

Although most reactions are mild, some adult patients are so sensitive that they respond even to the presence of an allergen in the recycled air on a plane. A few suffer an anaphylactic shock reaction. "There are some quite dramatic cases,'' says David Reading, director of the Anaphylaxis Campaign. "One member experienced a tingling sensation on eating a particular food and the following week suffered a near-fatal anaphylactic shock.''

In anaphylaxis, the immune reaction is so extreme that the blood vessels dilate and leak fluid into surrounding tissues, causing swelling, vomiting, diarrhoea, difficulty breathing and loss of consciousness. Trials are taking place of drugs that would switch off the dramatic immune response that takes place in allergy. Trying to avoid the problem foods is the best policy, but, in case this fails, people with an allergy should always carry an EpiPen, which contains epinephrine, and an asthma broncho-dilator. They should also wear a MedicAlert bracelet.


© The Telegraph Group Limited, London 2003