If you dismiss allergies as something that happen to other people, bear in mind that it can happen to anyone at any time, even if there's no known family history of such problems.
And as Allergy Awareness Week (April 25-May 1) approaches, experts are highlighting the fact that allergies are on the rise. Plus, while an estimated 21 million UK adults now suffer from at least one allergy, increasing numbers aren't just sneezing or experiencing irritating rashes or gut problems, but suffering from severe - potentially life-threatening - reactions.
In the last decade, food allergies have doubled, and the number of hospitalisations caused by severe allergic reactions has increased seven-fold - in fact, the charity Allergy UK notes that over the last two decades, there's been a 615% increase in hospital admissions for anaphylaxis, the most serious type of allergic reaction.
Like all reactions, anaphylaxis can occur out of the blue - and it's a medical emergency, causing severe breathing difficulties, a drop in blood pressure and swelling of the face and airways.
"Allergies can develop at any age," stresses Moira Austin, information manager for the Anaphylaxis Campaign. "Adult-onset allergy can mean adults who've eaten a food all their life, for example, can suddenly develop an allergy, and it can be a severe anyphylactic reaction."
There are a number of theories as to why this increase has come about, including the hygiene hypothesis, which suggests modern life is almost too clean, and our bodies are reacting adversely to substances they wouldn't have reacted to had they been exposed to more during childhood.
What you need to know
Despite the rising figures however, research suggests that more than half of people wouldn't recognise if someone was going into anaphylactic shock.
The main symptoms may occur alongside milder reactions, such as a red, raised and itchy rash, and while a reaction is usually classed as anaphylactic when the lungs, heart rhythm or blood pressure are affected, there may be other signs too.
For example, if someone starts vomiting after an insect sting, it can be the first indicator of anaphylaxis, because the sting has led to the release of chemicals which have travelled to the gut, and the vomiting may herald a more severe, widespread reaction.
"There are a whole range of symptoms," says Austin. "Generally speaking, anaphylaxis is an immediate reaction, with symptoms developing within minutes, or certainly within half an hour of being exposed to the allergen."
Antihistamines are usually effective for most milder allergic reactions but take too long to work when someone's having an anaphylactic attack - which should always be treated urgently with adrenaline, usually injected into the upper-outer thigh with an auto-injector adrenaline pen prescribed by a doctor (people known to be at risk of anaphylaxis are generally advised to carry one on them at all times).
The adrenaline starts to work within minutes, reducing swelling, relieving wheeze and improving blood pressure. It's also the only medicine which can stop cells releasing further chemicals into the blood as an allergic response.
An ambulance should be called too - even if an adrenaline injection has been given - and the person who's had the anaphylactic reaction should lie down, to help maintain blood pressure. Once in hospital, other medication, including oxygen, fluids, anti-histamines and corticosteroids, may be administered.
"Deaths from anaphylaxis are extremely rare, and tend to be when medication is given too late or isn't available at all. In the vast majority of cases, adrenaline starts to work on the system straight away and is very effective," says Austin. "It's extremely difficult to predict who's going to have a severe reaction, [and so] there's little sound advice that can be given to stop allergies developing.
"Just be aware of the symptoms, and if they're severe, dial 999."
What's going on?
Endless things can trigger an allergic reaction, but the most common causes of severe reactions include certain foods, especially peanuts, tree nuts and shellfish, bee and wasp stings and certain drugs.
In most allergic reactions, allergens in the offending substance lead to the release of chemicals - like histamine - into the tissues in a particular part of the body, like the skin or eyes. As a result, symptoms, such as a skin rash, or red, itchy eyes, are usually seen in this area. But in anaphylaxis, the chemicals are released into the bloodstream, causing serious symptoms very quickly.
It's not known why one person might have a mild allergic reaction, while another might go into anaphylactic shock. However, several factors can influence the severity of reactions, including exercise, heat, alcohol, the amount of allergen present, and even how food is prepared and eaten. Also, allergies can often run in families, and people with a history - or who have close relatives with a history - of asthma, eczema or hayfever may be more at risk.
Anaphylaxis is slightly more likely in people who've had a previous anaphylactic reaction, moderate, severe or uncontrolled asthma, a reaction after only skin contact with an allergen, or in people who have cardiovascular disease.
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