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What is Anaphylaxis?

Last updated on 5th April 2023

The UK has some of the highest incidences of allergic conditions in the world. A huge 44% of adults in the UK have at least one allergy.

The Anaphylaxis Campaign reported that hospital admissions for anaphylactic shock have increased by 27% in the last seven years. 4,756 adults and 1,707 children and young people were admitted to hospital in 2020 because of anaphylaxis.

These statistics represent a concerning increase in anaphylaxis and anaphylactic shock across the UK.

What is anaphylaxis?

Anaphylaxis is a severe and potentially life-threatening allergic reaction. The whole body is affected within seconds or minutes of exposure to the allergen, although in some cases anaphylaxis may not occur until hours after exposure.

Anaphylaxis can cause the immune system to release an excess of chemicals that can cause serious symptoms and reactions and result in the individual going into shock. This is known as anaphylactic shock and can be characterised by a sudden drop in blood pressure and the narrowing of airways which can block your breathing. The person may then become unconscious. If left untreated, anaphylaxis can develop into anaphylactic shock and result in serious complications or death.

Because anaphylaxis is such a severe allergic reaction, the individual requires immediate treatment and will likely need emergency intervention from a medical professional. Without the correct treatment, anaphylaxis can be fatal.

As anaphylaxis can be life-threatening, any person who is at risk of experiencing it should carry an adrenaline auto-injector with them at all times. It is also recommended that they wear a medical alert bracelet or carry an allergy card with them so that other people will be aware of their condition and can help to ensure that the individual receives the appropriate medical help.

If you have experienced anaphylaxis before, you are at higher risk of experiencing an anaphylactic reaction again.

Epi pen for anaphylaxis

What are the signs and symptoms of anaphylaxis?

When anaphylaxis occurs, the immune system releases a flood of chemicals which can cause a person to go into shock. Anaphylaxis usually develops suddenly and rapidly worsens.

Some signs and symptoms of anaphylaxis include:

  • A sudden drop in blood pressure (hypotension).
  • The narrowing of airways.
  • A weak or rapid pulse.
  • Feeling lightheaded or faint.
  • Breathing difficulties, for example, breathing that is fast or shallow.
  • A swollen tongue or throat.
  • Wheezing.
  • A fast heartbeat.
  • Confusion.
  • Anxiety.
  • Skin reactions, such as clammy, pale or flushed skin.
  • Hives or itching.
  • Nausea, vomiting or diarrhoea.
  • Collapsing or losing consciousness.

What are the different types of anaphylactic reactions?

There are several recognised temporal patterns of anaphylaxis:

Uniphasic Anaphylaxis

This is the most common type of anaphylaxis – with 80%–90% of anaphylactic reactions being uniphasic. This type of reaction usually peaks within an hour of the onset of symptoms and resolves within several hours, either spontaneously or after treatment. Anaphylaxis symptoms don’t usually return.

Biphasic Anaphylaxis

Biphasic reactions usually begin with symptoms that can range from mild to severe. This phase will then be followed by a period of time where the individual will be asymptomatic. The individual will then experience a subsequent return of symptoms, even though they have not been exposed to any more of the allergen. The anaphylaxis symptoms will usually have increased in severity and may include breathing difficulties and a rapid drop in blood pressure.

In most cases, the biphasic reaction occurs within a few hours of the initial reaction. However, in some cases, the biphasic reaction can occur 24–72 hours later. Individuals, particularly children, who have anaphylaxis may need to be observed in the hospital to ensure a biphasic reaction doesn’t occur. Between 10% and 20% of anaphylactic reactions are biphasic.

Protracted Anaphylaxis

Also known as persistent anaphylaxis, this is a reaction that lasts from several hours to several days without fully resolving. This type of reaction is less common and only occurs in a small number of cases. Individuals experiencing protracted anaphylaxis may need to be treated in hospital for several days.

A woman suffering from biphasic anaphylaxis

What to do if someone has anaphylaxis?

If someone near to you is showings signs of anaphylaxis, you should treat it as a medical emergency. Even if you are not completely sure the reaction is anaphylactic, you should treat it as though it is to reduce the likelihood of the reaction becoming life-threatening.

If someone is experiencing anaphylaxis symptoms, you should:

  • Use an adrenaline auto-injector if the person has one and you know how to use it correctly. If symptoms do not improve after five minutes, you should give another adrenaline auto-injector if possible.
  • Call 999 and request an ambulance. Make sure you tell the operator that you think the person is experiencing anaphylaxis.
  • Lie the person down and raise their legs to help blood flow to the vital organs and prevent a sudden drop in blood pressure. However, if a person is having difficulty breathing, you may need to sit them up to help them breathe. If the person experiencing anaphylaxis is pregnant, you should lie them down on their side.
  • If possible, remove the allergen trigger. This won’t be possible in some cases, for example, food allergens. However, allergens such as bee stings can be removed from the skin.
  • Put the person in the recovery position if they are unconscious.
  • Administer CPR if the person has stopped breathing or their heart has stopped.

What causes anaphylaxis?

Anaphylaxis is a type of allergic reaction that occurs when your body perceives an allergen as a potential threat and overreacts by releasing antibodies. The immune system is designed to protect your body from foreign substances, viruses and infection. If your immune system perceives an allergen as a threat, the antibodies that have been released will work to detect the allergen in future and protect you from it.

Next time you come into contact with the allergen, the antibodies will signal to your immune system which will release several chemicals, such as histamine, to fight the perceived threat and prevent the spread of a virus or infection. As there is no real virus or infection to fight, these chemicals can cause an inflammatory reaction.

Histamine and other chemicals can cause your blood vessels to dilate so that an increase of fluid can enter your tissues. If you experience anaphylaxis, the chemicals can affect many different systems in your body, and you can experience serious symptoms such as hypotension and breathing difficulties.

In the majority of people with allergies, coming into contact with an allergen will result in symptoms such as sneezing, itchiness, a rash, wheezing, stomach pain and red and itchy eyes. However, in people who experience anaphylaxis, coming into contact with the allergen can result in severe symptoms which can potentially be life-threatening.

As anaphylaxis is your immune system’s overreaction to an allergen, which is perceived as a threat, anyone who has experienced a severe allergic reaction is at a higher risk of anaphylaxis. Additionally, anyone who has had an anaphylactic reaction before, even a mild one, is at a higher risk of experiencing anaphylaxis in the future, if they are exposed to the allergen again.

You will not usually experience anaphylaxis the first time you are exposed to the allergen. However, once you have become sensitised to it, you can experience anaphylaxis at any time, at varying rates of severity.

Who is at risk of anaphylaxis?

There are certain risk factors that may raise the likelihood of someone having an anaphylactic reaction, including:

  • Previous anaphylaxis:
    If you have experienced an anaphylactic reaction before, you are more likely to have a reaction again. Future occurrences of anaphylaxis may be more severe than the first one.
  • Other allergies or asthma:
    People with allergies or asthma are at higher risk of anaphylaxis.
  • A family history of anaphylaxis or asthma:
    Having a close family member with anaphylaxis or asthma can increase the likelihood of you experiencing an anaphylactic reaction.
  • Other health conditions:
    Some health conditions linked with anaphylaxis include heart disease and mastocytosis.
Man with asthma

What are the triggers of anaphylaxis?

There are many different things that can trigger anaphylaxis. In most cases, the trigger is something that the individual is allergic to, but occasionally, anaphylaxis can be triggered by something else that the individual has no known allergy to.

The most common triggers of anaphylaxis are:

  • Food Allergies:
    The most common foods that result in anaphylaxis are nuts, peanuts, fish, shellfish, cow’s milk, eggs, wheat, soy, sesame and certain fruits.
  • Allergies to Medicines:
    Certain medications, including antibiotics, aspirin, pain relievers, general anaesthetic, non-steroidal anti-inflammatory medication, and certain contrast agents and dyes used in medical imaging tests can trigger an anaphylactic reaction.
  • Allergies to Insect Stings:
    Stings from bees, wasps, hornets and fire ants can all result in anaphylaxis. Biting insects can also cause anaphylaxis.
  • Latex Allergies:
    Latex is a type of rubber that is found in a variety of everyday items, including rubber gloves, balloons, elastic bands, condoms, swimming hats, some face masks and mattresses.
  • Exercise:
    Some people can develop anaphylaxis from exercise such as jogging, or even walking. Certain factors can raise the risk of anaphylaxis, for example, eating certain foods before exercising or doing exercise if the weather is particularly hot, cold or humid.
  • Idiopathic Anaphylaxis:
    This is when there is no known trigger. In most cases, if you cannot identify what triggers your anaphylaxis, you will likely undergo certain tests to identify the trigger or allergen. If all your skin tests are negative, you have absent or undetectable serum-specific IgE levels, and the trigger cannot be identified by reviewing your history, diet and lifestyle, you will likely receive a diagnosis of idiopathic anaphylaxis.

What increases the risk of a severe anaphylactic reaction?

The severity of an anaphylactic reaction can vary depending on several factors. There are certain things that may make you more vulnerable to severe anaphylaxis and at these times, you should take particular care to avoid any known allergens.

Factors that can increase the risk of a severe anaphylactic reaction are:

  • If you have poorly controlled asthma or are currently experiencing symptoms of asthma.
  • If you have or have recently had an infection.
  • If you have exercised in the time just before or just after you came into contact with the allergen.
  • If you are currently experiencing hay fever.
  • If you have been drinking alcohol.
  • If you are currently taking a non-steroid anti-inflammatory drug (NSAID), for example, aspirin or ibuprofen.
  • If you are experiencing high levels of stress.

How to prevent anaphylaxis

There are certain measures you can take that may help you to prevent anaphylaxis.

Although these may not be 100% effective, they can help to reduce the risk of you having an anaphylactic reaction:

  • Identify your triggers:
    You may be able to do this with help from your GP or you may be referred to a specialist allergy clinic to undergo allergy tests.
  • Avoid your triggers:
    Once you know what your triggers are, you can avoid them as much as possible. If you have food allergies, always check labels carefully and inform restaurant staff of your allergies. If you are allergic to stinging insects, preventative measures such as wearing long-sleeved clothes, not wearing bright colours, not wearing perfumes, and moving slowly away from the insect can help to prevent stings.
  • Carry two adrenaline auto-injectors with you at all times:
    You should ensure your auto-injectors are in date. Even if you are not completely sure whether you are experiencing anaphylaxis, you should still administer the injection. You could also train people close to you on how to administer the adrenaline auto-injectors.
  • Wear a medical alert bracelet or necklace or carry a card:
    This is particularly recommended if you are allergic to medications. If you are admitted to the hospital in an emergency, the medical staff will know not to give you the specified medications, which will prevent an allergic reaction.
  • Be aware of allergy symptoms:
    You may not experience the same symptoms every time so being aware of all possible anaphylaxis symptoms can help you to identify signs of anaphylaxis quickly. This may prevent you from experiencing a severe anaphylactic shock.
Knowing triggers

Treatment and diagnosis of anaphylaxis

To diagnose anaphylaxis, your doctor will likely ask you questions about any previous allergic reactions you have had and any known triggers.

To confirm an anaphylaxis diagnosis, a doctor will likely give you a blood test that will measure the amount of tryptase in your blood. Tryptase is an enzyme that remains at an elevated level for up to three hours after an anaphylactic reaction. You may also be given a skin test or blood test to check what you are allergic to.

Anaphylaxis is usually treated with an adrenaline auto-injector. The adrenaline works by quickly opening up the person’s airwaves, reducing any swelling and raising their blood pressure.

Adrenaline auto-injectors are most effective when administered quickly. It should be given as soon as possible if someone begins to show signs of a severe allergic reaction. Adrenaline auto-injectors can be administered by the person experiencing anaphylaxis, or by someone else.

The sooner the adrenaline is administered, the easier it is to reverse severe symptoms and stop an anaphylactic reaction from becoming life-threatening.

There are three main types of adrenaline auto-injectors in the UK. Each one is used in a different way.

1. Auto-injector.

2. Jext.

3. Emerade.

Anyone who is at risk of anaphylaxis should carry two in-date adrenaline auto-injectors with them at all times. You should ensure you know how to administer the injection, but instructions are provided on the side of each injector in case you forget how to use it, or someone else needs to administer the adrenaline.

Anyone experiencing anaphylaxis should be placed or place themselves in the correct positioning to help support their breathing and blood pressure.

  • Lie flat with their legs raised in the air.
  • Lie on their left side if they are pregnant.
  • Sit up for a short period if they are having breathing difficulties and then lie down as soon as possible.

Anyone experiencing anaphylaxis should avoid any sudden changes to their upright posture as this could result in a dangerous fall in their blood pressure. Because of this, standing or sitting up should be avoided as much as possible during an anaphylactic reaction.

If a person in anaphylaxis stops breathing, or the heart stops, you should perform cardiopulmonary resuscitation (CPR) as soon as possible.

If you have experienced anaphylaxis, you should go to the hospital for observation as soon as possible. You may need to call 999 and request an ambulance, or if your symptoms are less severe, you could visit the Accident & Emergency department of your local hospital.

The hospital will likely use a blood test to ensure you receive the correct diagnosis of anaphylaxis.

You will likely need to remain in the hospital for at least six hours to ensure your symptoms are under control and are not likely to return. The more severe your anaphylaxis symptoms, the longer you will likely have to spend in hospital.

While you are in hospital you may be given intravenous fluids to help raise your blood pressure, an oxygen mask to aid your breathing, and medication such as antihistamines or steroids to relieve your physical anaphylaxis symptoms. You may also need to take antihistamine and steroid tablets for the days after you have experienced anaphylaxis.

There is currently no cure for allergic reactions or anaphylaxis. Instead, your GP or an allergen specialist will teach you techniques to help manage your allergies and reduce the likelihood of anaphylaxis occurring in the future.

What support is available?

Individuals who experience anaphylaxis, or the friends and family of a person who experiences anaphylactic reactions, can benefit from accessing support. Your GP can refer you to an allergy specialist who can offer medical support and arm you with the knowledge and skills you need to protect yourself and your loved ones from anaphylactic shock, as well as teach you how to recognise the anaphylaxis symptoms.

However, in many circumstances, the individual may desire additional support. The Anaphylaxis Campaign is the only UK-wide charity that solely supports those at risk of anaphylaxis and their family, friends and carers. They offer online training and education to individuals, healthcare professionals, schools and colleges about anaphylaxis.

The Anaphylaxis Campaign also offers local support groups for those affected by anaphylaxis and it campaigns for improved allergy services. You can visit the Anaphylaxis Campaign website, email them at, or call their helpline on +44 (0) 1252 542029.

Alternatively, you can seek support from Allergy UK. Allergy UK supports people with any type of allergy, regardless of the severity. They offer help and support, information and resources about allergies. You can visit Allergy UK’s website, or call their helpline on 01322 619898.

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About the author

Nicole Murphy

Nicole Murphy

Nicole graduated with a First-Class Honours degree in Psychology in 2013. She works as a writer and editor and tries to combine all her passions - writing, education, and psychology. Outside of work, Nicole loves to travel, go to the beach, and drink a lot of coffee! She is currently training to climb Machu Picchu in Peru.

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