It's nutritious, it's tasty and it's the fruit of choice for a growing number of children. But for nine-year-old Christian Moore the mere sight of a kiwi fruit is positively sickening. It isn't just that he doesn't like it or that it doesn't agree with him. It could kill him - and once almost did.

Christian's mother, Birna Helgadottir, knew he was allergic to milk and egg when he was very young, but it wasn't until he was five that his much more serious kiwi fruit allergy came to light. "It was 'K' day at school and all the children were asked to take in something starting with the letter k. A lot of people had KitKats but Christian couldn't have one because of his allergies, so someone gave him a kiwi fruit thinking that would be a good substitute.

"The school phoned up to say Christian had eaten something that he had reacted badly to and when I got there his face was completely swollen, his lips were misshapen, his eyes sunk into the back of his head. He was in a bad way and it was very scary."

Because of his other allergies Helgadottir had antihistamine with her and he recovered.

His second brush with the fruit was even more frightening: this time, Christian didn't actually eat it - he just ate something that had been cut using a knife previously used to slice a strawberry and kiwi pavlova. "This time was even worse, and I knew how serious it could be. We rushed him to hospital and they gave him adrenalin within 30 seconds of getting in through the door," says his mother.

Not surprisingly, the family is now careful not to let Christian have any contact with kiwi fruit. "Even seeing pictures of it frightens him, which is understandable when you think he could have died," says Helgadottir. At friends' houses, she checks the fruit bowl when she arrives and hides any kiwi fruit rather than take chances.

The severity of Christian's reaction to kiwi fruit is unusual, but he is one of an increasing number of children who are allergic to it. Doctors at Southampton University are carrying out the biggest study yet into the phenomenon, and interim results last week showed that two thirds of children became ill the first time they ate kiwi. The study - involving 300 people, 80 of whom were children - found reactions that included tingling and itching in the mouth or throat and swelling of the tongue.

As with all allergies, what happens when Christian comes into contact with kiwi is that his immune system makes a massive, and in his case potentially fatal, mistake. Eating kiwi fruit represents no threat or risk whatsoever to Christian's health and well-being, but that is not how his lymphocytes perceive it. For some reason that is not yet understood, his body's defence system goes on major alert when they come into contact with the proteins in the kiwi fruit: and having wrongly identified the proteins as a danger, they compound the error by setting in motion a chain of events that are life-threatening. Lymphocytes are programmed to neutralise dangerous proteins but in an allergic reaction they damage the structure of some of the cells they are attacking, and these release a surge of a chemical called histamine which dilates the small red blood vessels. Blood then escapes into surrounding tissue, causing the typical symptoms of an allergic reaction: red and itchy skin, swelling and so on.

The most severe allergic reaction, anaphylactic shock, is a kind of no-holds-barred, explosive response to an allergen and involves a whole range of body systems including the skin, intestines, respiratory system and cardiovascular system. People at risk of anaphylactic shock must carry a dose of adrenalin in the form of an EpiPen, a device that allows it to be delivered fast, straight into the skin to reverse the response.

Studies show that all allergies are on the rise, with one in four people in the UK now affected at some time in their lives. The problem has grown so quickly that NHS resources are ill-equipped to deal with it, which is why the Royal College of Physicians will publish a report tomorrow, calling for major improvements in the provision of specialist services for people with allergies. So far, there is only a handful of specialist clinics, all in south-east England.

It is not known why allergies are becoming so widespread: with some indivdual allergies, such as kiwi fruit, experts have pointed to the fact that a luxury item has become, in a short time, a common foodstuff. In 2001, around £21.6m-worth of the fruit was imported into Britain, mainly from New Zealand, but also from Italy, South Africa and Chile, and according to a recent Mori poll, one in 10 children said it was their favourite. Not only do children like the taste, kiwi fruit is exceptionally nutritious and packed with vitamins C and E, potassium and fibre.

Although only introduced to British supermarkets in the 1960s, kiwi fruit has a history stretching back 700 years in China. Seed samples were taken by British missionaries to New Zealand at the beginning of the 20th century, where it became known as the Chinese gooseberry and was soon widely grown. In the early 1960s it was introduced into the US and was cultivated in California, where it was renamed the kiwi fruit after the New Zealand national emblem. Although we think of kiwi as a single variety in the UK, there are more than 400 different strains and researchers are trying to find out whether certain varieties are more allergenic than others. What is known is that, for most people with an allergy to it, cooking renders it harmless

One of the many mysteries surrounding allergies is why some foods produce widespread allergic reactions in certain parts of the world, but not in others.

Kiwi fruit is a known allergen in New Zealand - the first cases there were reported in the early 1980s. In the UK, one child in 70 is allergic to peanuts but in south Asia, where peanuts are much more widely used than in Europe, peanut allergy is not widespread - one theory is that roasting peanuts, which is more common in the west than in Asia, increases their allergenic properties. Rice, a staple in Asia, is a common allergen there - especially in Japan where it is consumed in large quantities - but allergy to rice is little known in Britain.

According to Dr Shuaib Nasser, consultant physician in allergy and asthma at Addenbrookes hospital in Cambridge, the prevalence of kiwi fruit allergy in this country probably has less to do with its availability and more to do with its protein make-up which closely resembles that of latex, or rubber. Latex allergy is hugely on the increase, he says, particularly among healthcare workers. "Since the mid-1980s we have been using latex gloves far more often because of worries about HIV, and because we were using so many, we started importing cheap ones in which the latex leaches off the surface, sticks to the skin and sensitises it," he says.

Some allergy organisations also warn that children who have had a lot of surgery or been hospitalised frequently may have become sensitised to latex because of heavy use of the gloves in their treatment: anecdotally, they say many of the children they see with kiwi fruit allergy have a history of heavy medical treatment.

Latex - like nuts, another common allergen - is rich in proteins, and it seems that protein-filled substances are much more likely to prompt an adverse reaction. "We know that latex allergy and fruit allergy are closely related - up to 50% of people with a latex allergy are fruit allergic too," says Nasser. "On the whole, fruit is easier to avoid than nuts, and in most people the reaction is less severe than with nuts."

On the broader question of why people today seem more likely to experience allergic reactions, Nasser believes the answer may lie in our over-hygienic environment. Western city dwellers in particular, he says, simply don't face the risks to their health that their parents and grandparents did and, as a result, immune systems - particularly those of children - are left idle. Unfortunately, an idle immune system seems to spell trouble: it is almost like the bright child who, finding herself without proper stimulation in the classroom, turns her attention to mischief-making instead.

Maureen Jenkins, an allergy nurse consultant, says that people tend to be allergic to groups of things: if you are allergic to one substance, the chances are high that you are allergic to similar ones. But the most severe reactions, she points out, are often a response not just to one allergen but to a particular set of circumstances, including the allergen: it might be that for some people, for example, an allergen will only produce anaphylactic shock in conjunction with a second factor, such as having recently taken strenuous exercise.

Does keeping a child out of contact with known allergens protect them from developing an allergy in future? Official advice is that parents should avoid giving nuts and nut products to pre-school children for this reason but, says Nasser, it is a precaution: there is no scientific evidence to back it up. As far as kiwi fruit is concerned, he says there is no reason to keep it from young children: it is still a rare allergy, and there is no suggestion that early exposure makes any difference to whether a child will react to it or not.

Danger zones

· Peanut allergy is on the increase - one child in 70 now reacts to them

· Other common food allergens are soya; tree nuts (walnuts, brazil nuts, almonds etc); fish; milk; eggs; shellfish and wheat

· Latex allergy is spreading - although not a foodstuff it's closely related to fruit allergies

· Fruit allergies in children are on the up - in particular apple, banana, avocado and pear

· Sesame seeds are another allergen on the rise - they are eaten in products like houmous