Three years ago, Liz Newman was in a pub in Glasgow, having a drink with friends. All was going well, until a woman nearby opened a packet of dry roasted peanuts. Though the stranger was so far away that you wouldn't be able to conduct a conversation with her at normal volume, tiny nutty particles managed to make their way across the room and down Newman's gullet.

Within minutes she was unconscious on the floor. She was having an anaphylactic shock, a severe allergic reaction that occurs rapidly and causes a life-threatening response involving the whole body. Her lips and neck swelled as hives appeared suddenly on the lining of her throat, blocking her airways. Her blood pressure dropped dramatically and she began to suffocate. If it hadn't been for the quick action of her friends, who called an ambulance and injected her with the adrenalin phial she carries at all times, Newman would have died. That was when she knew that she could definitively add nuts to her personal list of banned substances.

Because it's not just nuts. Newman, 23, who describes herself as "one of the most allergic people in the country", has what is known as Oral Allergy Syndrome. She becomes quickly and violently ill if she comes into contact with any allergen from an almost endless catalogue that includes: raw fruit, raw vegetables, cooked fruit, fruit juices, some cooked vegetables, ham, cumin, nutmeg, fennel, chick peas, french fries, rye, anything lactose derived (excluding cheese), some beauty products, rubber gloves, condoms, trees, tree pollen, wasps and wasp venom.

"I can't even travel on public transport any more, or at least I try not to," says Newman. "On trains especially, they recycle the air, if anyone on board has been eating something I'm allergic to, there's a chance it will get into the ventilation system and I could breathe it in, and go into shock."

Anaphylaxis can set in if Newman even comes into contact with something that has indirectly encountered her allergens. For example, if she were to go to a restaurant and a waiter had laid cutlery on the table having just touched raw fruit and she put the fork in her mouth; or if her boyfriend Matthew had eaten something "not Liz-friendly" (the categorisation used by her friends and family) and not washed his mouth out properly before giving her a kiss.

The strange thing is, until she was 13, Newman could eat what she liked and spend time outside to no ill effect. Then suddenly, in spring 1995, she developed severe asthma and hayfever. Soon after, she started having problems tolerating pears, and then apples and carrots, and before long any raw vegetables and most fruits were off the menu. "Sometimes," she says, "just walking through the fruit and veg section of the supermarket I could feel my throat tightening."

It wasn't until Newman was 19 and in her first year of an archaeology degree at the University of Kent in Canterbury that the diagnosis came. "I went to see an allergy specialist at a private clinic in Kent," recalls Newman, "and she gave me a skin prick test for 12 random items. I was allergic to everything and one of the tests, for peppers, caused an 8cm-wide swelling on my arm. That was when I was warned that I was highly likely to develop anaphylaxis. Peppers are still my worst allergen today."

Her first anaphylactic shock occurred when she was driving back from a weekend in Dorset. Since then, she has had an estimated 100 shocks - that's almost one a fortnight. The last one occurred three weeks ago. She was cooking dinner alone at home, a simple meal of rice and peas, and undercooked the peas by a minute or so.

Frustratingly, however, no one can explain to Newman why she has the condition. Neither of her parents have severe allergic reactions and she has never been able to pinpoint what might have triggered it 10 years ago.

She is not alone. Despite the fact that 15m Britons now claim to suffer from an allergy (more than in any other country), scientists have been unable to prove what has caused the epidemic. Dr Mike Matthews, a retired GP and chairman of the charity AllergyUK, explains that the most popular theory is the hygiene hypothesis: "We're too clean in our modern society. In our bid to kill 99% of the bacteria around, particularly to protect our children, we are upsetting our immune system. The vast majority of bacteria are not harmful, but rather stimulate the production of antibodies which protect against disease. Killing these off leaves us wide open."

For Newman, not knowing why she suffers is a bitter pill to swallow. "Despite it, I've had to come to terms with the fact that this thing could kill me," she says. "It has totally changed the way I look at life. Before this all happened, I thought I could do absolutely anything. I assumed that I would travel the world as an archaeologist. I never thought I'd still be here in Canterbury."

Still, at least she lives in the south-east. This means she has relatively easy access to one of the six specialist centres in England (there are none in Scotland or Wales), which are run by allergists providing a full-time service. Although there are about 100 other clinics in the UK providing services in allergy, they are mainly run by non-specialist physicians, paediatricians and immunologists, and can generally only provide a part-time service. In all cases, long waiting lists apply. There are only 10 full-time NHS specialist allergy consultants in the country: in Sweden there are 90 and in Germany 500 - for children alone. This has led to a proliferation in so-called "alternative" allergy clinics and diagnostic services, some of which offer unvalidated tests and inappropriate advice for allergy sufferers.

So why this dearth of expertise? Dr Matthews thinks it's largely due to an educational problem within medicine. "The way British medicine has developed over the past 100 years, allergies haven't been on the syllabus, so doctors don't know how to deal with them."

It is this ignorance, say experts, that causes doctors to not treat allergies seriously - it is still a widely held view among psychiatrists that allergies are "all in the mind".

"Try telling that to someone who suffers from anaphylaxis," says Matthews. "I saw something on BBC Breakfast News that made me so angry," says Newman. "[The presenter] Natasha Kaplinsky was talking about allergies, and said something like, 'isn't it just people being a bit namby-pamby?' People like me are not being 'namby-pamby'. We have a life-threatening condition and it's not our fault." Allergic to Everything will be shown on Channel 4 on Thursday at 9pm.

· Useful links: Allergy UK, www.AllergyUK.org helpline: 01322 619898. The Anaphylaxis Campaign, www.anaphylaxis.org.uk