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Are women more allergy-prone?

In adult life, women are certainly more likely than men to report having allergies and intolerances to food. Intolerances differ from allergies (see 鈥淣ot an allergy鈥�, below), but they are often grouped together in studies. Last year, a paper examining 2.7 million health records found that 4.2 per cent of women have food allergies or intolerances, compared with 2.9 per cent of men. And this isn鈥檛 just a reflection of the apparent rise in gluten intolerance: the most common problem food for both men and women was shellfish, followed by certain fruits and vegetables.

It is possible that women simply pay more attention to what they eat and their health, so are more likely to notice if they have a reaction, but biological factors seem to be involved too. For example, men have higher levels of a certain type of antibody thought to help block allergic reactions.

And hormones almost certainly play a role because the gender difference in allergies switches at adolescence. Before puberty, boys are affected by allergies twice as much as girls.

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Can you grow into and out of allergies?

Adult-onset hay fever often comes as a surprise to those newly affected听鈥� but it is true, you really can develop fresh allergies throughout your life. Just because you have never been allergic to pollen or peanuts, doesn鈥檛 mean you never will be. The flip side is that allergies can sometimes fade away, although this is rarer once you have entered adulthood.

You are most likely to grow out of an allergy as a child. One study of more than 40,000 children in the US found that around , usually by the age of 6. Egg, milk and soy allergies are those most often left behind. About .

When young children have eczema听鈥� a skin condition linked to allergies听鈥� this often improves in late childhood only for it to be replaced by asthma. The same children are then likely to get hay fever as teenagers, which听tends to subside around their mid-20s. This procession of changing allergies is known听as allergic march. Some symptoms from the old allergies tend to continue at some听level. 鈥淚t often doesn鈥檛 go听away, it听improves,鈥� says Syed Hasan Arshad at the University of Southampton, UK.

The reason for this progression is still a mystery. 鈥淚 wish I knew why it changes,鈥� he says. 鈥淚f I did, I could find a cure for allergies.鈥�

Drugs, hormones, other medical conditions and exposure to smoke and other air pollution may all play a role in changing allergies, says Neil Kao, of the Allergic Disease and Asthma Center, in South Carolina. After the menopause, women no longer experience allergies at a higher rate than men, he says.

But if you think you have recently beaten an听allergy, the chances are you have just been exposed to less of the allergen responsible. 鈥淕eographically moving is the single most common reason why people think they鈥檝e outgrown their allergies,鈥� says Kao.

What of the oft-cited idea that allergies change every seven years? It is really a lot less predictable than that. 鈥淓very person鈥檚 immune tolerance levels fluctuate throughout his or her life, spontaneously and naturally. This is dictated by their genes,鈥� says Kao.

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Do allergies come in groups?

If you know someone who can鈥檛 eat shellfish and also complains about dust, they aren鈥檛 (necessarily) just being picky. Allergies can come in a gang, and some combinations are more common than others.

This is because allergies are caused by IgE antibodies recognising harmless molecules as a threat, and allergens with a similar structure can trigger responses in the same person.

This often occurs in two phases. Typically, someone who developed an allergy to a common allergen such as pollen as a child will begin reacting to similar chemical structures in food later in life. This is called cross-reactivity and can lead to oral allergy syndrome: the rapid onset of local symptoms such as itching, tingling and swelling after eating certain foods. may have it. 鈥淚t鈥檚 a fairly common type of allergy that isn鈥檛 dangerous, but it can be quite annoying,鈥� says Arshad.

Tree pollen can cause allergies to a range of fruits, nuts and vegetables, such as apples, almonds and carrots. Ragweed allergies are associated with problems with melons, bananas, tomatoes and cucumbers, while mugwort pollen can trigger allergies to herbs and spices such as coriander, parsley, cumin and fennel seed. Many other groupings occur, including latex-related allergies to bananas, apples, potatoes and tomatoes.

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Not an allergy

Allergies are caused by the immune system unnecessarily responding to harmless molecules. But not all rashes, swellings or breathing difficulties are a sign of one.

Food intolerance
Food intolerances can involve bloating, wind, diarrhoea, itching, skin rashes and abdominal pain. These symptoms come on more slowly than those of food allergies, usually a few hours after eating certain things. For instance, some people report such symptoms after eating gluten, despite not having coeliac disease, an autoimmune disorder triggered by this protein component of cereal grains.

Chemical sensitivity
As many as a quarter of people in the US say they are sensitive to fragranced products such as deodorants and air fresheners. People with these chemical sensitivities most commonly experience asthma when exposed to certain chemicals, although others report migraines,听skin complaints or听shortness of breath.

Nocebo effect
Contrary to what some believe, allergies are not psychosomatic. However, another psychological phenomenon can make people experience allergy-like symptoms. The nocebo effect is a negative twist on the placebo effect, resulting in some people experiencing nasty symptoms even if a substance has no adverse effect on their body. One study found that with the drug. Another study, in which researchers gave allergenic or placebo foods to children with reported food allergies, found that placebo foods , including rashes, hives, diarrhoea and vomiting, nearly 13听per cent of the time.

Rhinitis
Rhinitis is inflammation of the inside of the nose, causing symptoms such as sneezing and a听blocked or runny nose. This can be a response to allergens such as 听pollen or dust mites, but it . Causes include humidity, extreme temperatures, viruses and 听exposure to air pollution such听as smoke.

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This article appeared in print under the headline 鈥楢llergy explosion鈥�