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What is anaphylaxis?

Anaphylaxis is a serious allergic reaction that affects the whole body and can be life-threatening, so it always needs emergency treatment.

Reactions usually begin within minutes and progress quickly, but can sometimes start two to three hours after you’re exposed to the allergen.

If you are diagnosed with a serious allergy and are at higher risk of anaphylaxis, your doctor may prescribe adrenaline to use in an emergency.

Download our anaphylaxis factsheet

Download the factsheet

What causes anaphylaxis?

The most common allergens that can cause anaphylaxis are foods such as peanuts, tree nuts, milk, eggs, shellfish, fish and sesame seeds, although many other foods have been known to trigger it. Some people react even to tiny amounts of food.

Other causes include wasp or bee stings, latex (natural rubber latex) and medicines. In some people, exercise can trigger anaphylaxis – either on its own or if you exercise around the same time you’re exposed to other allergens.

Sometimes people have a reaction and the cause can’t be found. This is called ‘idiopathic anaphylaxis’, which means the cause is unknown.

 

What are the signs and symptoms of anaphylaxis?

Most healthcare professionals consider an allergic reaction to be anaphylaxis when it involves difficulty breathing or affects the heart rhythm or blood pressure. Any one or more of the following symptoms may be present – these are often referred to as the ‘ABC’ symptoms.

 

In extreme cases there could be a dramatic fall in blood pressure. The person may become weak and floppy and may have a sense of something terrible happening. Any of the ABC symptoms may lead to collapse and unconsciousness and, on rare occasions, can be fatal.

AIRWAY

Swelling in the throat, tongue or upper airways, hoarse voice, difficulty swallowing

BREATHING

Sudden onset wheezing, breathing difficulty, noisy breathing, persistent cough

CIRCULATION

Dizziness, feeling faint, sudden sleepiness, confusion, pale clammy skin, loss of consciousness or collapse

 

 

Other symptoms

Mild to moderate symptoms may include: 

  • a red raised itchy rash (known as hives or urticaria) anywhere on the body  
  • swelling of the face, lips and/or eyes  
  • a tingling or itchy feeling in the mouth 
  • mild throat tightness 
  • stomach pain, vomiting or diarrhoea 

These symptoms can also happen on their own. If you don’t have the ABC symptoms, the reaction is likely to be less serious and is not the same as anaphylaxis, but watch carefully in case ABC symptoms develop.

Why does anaphylaxis happen?

An allergic reaction (including anaphylaxis) happens when the body’s immune system wrongly identifies a food or substance as a threat. When this happens, the body releases chemicals such as histamine in response. It is the release of these chemicals that causes the allergic symptoms.

Getting a diagnosis

If you have had an allergic reaction in the past, visit your GP who can refer you to a specialist allergy clinic if needed. They can find a clinic in your area from the British Society for Allergy and Clinical Immunology (BSACI).

It’s important to get a referral even if your symptoms were mild because it can be hard to tell if future allergic reactions could be more serious.

Once you get a referral, the consultant will discuss your medical history and symptoms with you. They might suggest skin prick tests and blood tests, and for food allergies, food challenge tests, to help diagnose the allergy and work out how serious it may be.

More about allergy testing

What can mean you’re at higher risk?

Some clues that you might be at higher risk are:

  • you have already had a serious reaction, with any of the ‘ABC’ symptoms
  • you have asthma, especially if it is not well controlled
  • you have reacted to a tiny amount of your allergen.

 

If you have asthma and it is not well controlled, this could make an allergic reaction worse. Make sure you discuss this with your GP or allergy specialist and take any prescribed medicines.

The National Institute for Health and Care Excellence (NICE) have produced guidance on assessment and referral.

 

advising about adrenaline for allergies
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Treating symptoms

If you have mild allergic symptoms, you may be prescribed antihistamine medicine that you take by mouth. But if you are at risk of a serious allergic reaction (anaphylaxis), you may be prescribed adrenaline – the emergency medicine used to treat anaphylaxis. It is also known as epinephrine. 

Because anaphylaxis can happen very quickly, adrenaline is available in different forms that are designed to be easy to use. It’s important to know exactly how and when to use your prescribed adrenaline.  Healthcare professionals can show you how to use it, and there are also resources such as practice devices and videos on manufacturer websites. 

Options currently available on prescription in the UK include: 

  • Adrenaline auto-injectors (AAIs) – such as EpiPen and Jext. 
  • Intranasal adrenaline – EURneffy, a needle-free nasal spray. 

You must carry two in-date forms of prescribed adrenaline at all times as a second dose may be needed if symptoms do not improve after five minutes or get worse.  

Find out more about adrenaline. 

Read more about what to do in an emergency

What can I do to protect myself?

  • If you have asthma as well as allergies, make sure your asthma is well-managed and regularly reviewed.
  • If you have been prescribed adrenaline, make sure you carry two devices with you at all times and know how to use them.
  • Think ahead. Have an emergency allergy action plan and make sure those around you understand it.

Allergy Action Plans for children

The British Society for Allergy and Clinical Immunology (BSACI) produce template Allergy Action Plans for children. There are three plans available; a generic plan for individuals assessed as not needing an adrenaline auto-injector, and a personal plan for individuals prescribed an EpiPen or Jext pens.

Download the BSACI Allergy Action plans here

Allergy Action Plan for adults

Anaphylaxis UK has developed a template Allergy Action Plan suitable for adults.

Download our Allergy Action plan for adults here

What to do in an emergency

Find out more

Biphasic anaphylaxis

If you have anaphylaxis, you will need to stay in hospital after a serious reaction. This is because a second wave of symptoms can develop – known as a biphasic reaction.

It is not known exactly how many people will have biphasic reactions. Studies suggest symptoms return in between one and 20 out of every 100 people who have had anaphylaxis.

Around half of biphasic reactions happen within six and 12 hours of the first reaction. They are less common with food allergies than other allergies.

 

Does the risk of anaphylaxis get less over time?

There is no evidence that the risk of anaphylaxis reduces over time, but some people do outgrow their allergies. This is more common in young children, particularly those who are allergic to cow’s milk, egg and wheat.

What is mastocytosis?

In most cases of anaphylaxis there is a trigger, such as a food, drug or insect sting. But anaphylaxis can also happen in people who have a very rare condition called mastocytosis.

Mastocytosis is caused by too many mast cells (a type of blood cell that forms part of the immune system) collecting in the tissues of the body. These are the main cells that release histamine and other chemicals that cause allergic symptoms. If you have mastocytosis, it’s important to get a diagnosis.

Key messages  

  • If you think you might have an allergy, see your GP.
  • If you are prescribed adrenaline, carry two devices with you at all times.
  • Make sure you know how to use your adrenaline and get a trainer device to practise with.
  • Use your adrenaline as soon as you notice any signs of anaphylaxis – know the ABC symptoms so you can act quickly.
  • Learn what to do in an emergency.

Download our anaphylaxis factsheet

Download the factsheet