Anaphylaxis, or anaphylactic shock, is a sudden catastrophic allergic reaction that involves the whole body. It usually occurs within minutes of exposure to the offending allergen (insect stings, nuts and medication being the commonest causes).

By Dr. Adrian Morris

pillsThe first documented case of anaphylaxis was in 2641 BC, when King Menes of Egypt died from a Wasp sting.  While the first fatal reaction to peanuts was described by a Canadian researcher Dr Evans in 1988.

About 1 in 500 people attending hospital Emergency Units come because of some form of anaphylactic reaction and the number of new cases in developed countries seem to be rapidly increasing. Possible changes in our diet and environment have been implicated (nut anaphylaxis was first documented only 20 years ago). Approximately 150 food allergy deaths are reported in the USA annually.

Anaphylaxis causes

Why some people develop anaphylaxis and others not, is difficult to explain.  But it usually happens to people who are known to have allergies and particularly those who suffer with brittle asthma.

The most common cause of anaphylaxis in the community is from eating a food to which you are allergic such as nuts, peanuts, eggs, mammalian milk, soya, wheat, fish and shellfish.  These 8 foods account for 90% of cases of food induced anaphylaxis.  Peanuts,Sesame seed and tree nuts (such as Brazil nuts, Hazelnuts, Cashew, Pistachio and Walnuts) are the foods most likely to provoke a reaction.

Even eating a tiny amount of a particular food can cause anaphylaxis. Some people are so sensitive that breathing in the food essence can trigger a reaction (as in a Restaurant when the person at the next table is eating fish or kissing a person who has recently eaten peanuts).

Allergy to venom from wasp stings can cause anaphylaxis as can allergy to latex and drugs such as penicillin, codeine and aspirin. Bee sting allergy is less common in the UK.

In the hospital context, the majority of anaphylactic reactions are to medicines such as muscle relaxants, antibiotics, xray contrast medium and injectable medications including anaesthetic agents.  Latex allergy and anaphylaxis is becoming an ever increasing problem, particularly in health workers and children who have frequent operations and are sensitised by exposure to Latex in surgical gloves, drip sets and catheters.

Some people may develop anaphylaxis after eating certain foods such as celery, shrimps, wheat, apple, hazelnut, squid and chicken and then exercising shortly after ingesting the food – triggering Food Related Exercise Induced Anaphylaxis (FREIA).

Often we never find the cause of the anaphylactic reaction and in 1978 the term “Idiopathic” Anaphylaxis was coined to describe this group of people which accounts for about half of all cases of anaphylaxis.  However new and previously unrecognised allergens such as lupin flour (in European pastries) and celeriac should be considered.

Anaphylaxis symptoms

Anaphylaxis comprises a group of symptoms and features, which in combination lead to a generalised severe allergic reaction with respiratory difficulties and circulatory shock.

The initial reaction is swelling and itching of the area where the allergen has entered. Food for example, initially causes swelling and itching of the mouth and throat while a wasp sting will cause intense itching and swelling at the sting site.

A generalised reaction then rapidly follows with an itchy rash that spreads over the whole body.  The face and soft tissues begin to swell and breathing becomes difficult. The scalp, palms and soles of feet become intensely itchy.

The person becomes very agitated with a “feeling of impending doom”, tightening of the throat and chest occur, while the blood pressure may begin to drop and the victim then loses consciousness.

These symptoms develop very rapidly within a few minutes of coming into contact with the allergen. Most anaphylactic reactions occur within an hour of exposure to the causative allergen.

Anaphylaxis treatment

Anaphylaxis needs to be treated as a matter of urgency as the symptoms of respiratory obstruction and shock develop rapidly. Emergency treatment consists of an injection of a drug called adrenaline or epinephrine, which raises blood pressure, relieves breathing difficulties and reduces swelling. Once adrenaline has been given, people normally recover very quickly. They should also immediately take a dose of antihistamine and will need a short course of cortisone (steroid) tablets to prevent a recurrence of the reaction a few hours later (biphasic response).

If you have had a previous and severe anaphylactic reaction, make sure you use the Adrenaline Injector (Epipen, Emerade or Jext) straight away with any future reactions – as any delay puts you at far greater risk for collapse and death.  There are currently 3 brands of adrenaline auto injectors available in the UK Emerade, Epipen and Jext pens, the Anapen is no longer available. Although many people carry an adrenalin auto injector, the most common cause of death is failure to use it!  There are two types of injector, one for children and another for adults.  The injector is easy to use and is activated by pressing firmly over the front of the thigh muscle.  Your doctor should issue you with a “trainer” Epipen, Emerade or Jext to practice self-administration.

If you have always experienced milder attacks with minimal or no breathing difficulties we usually recommend that you immediately take antihistamine medication and monitor the situation for a few minutes before giving adrenaline.  Milder symptoms then usually resolve over the next half hour.  Always make sure you have someone with you who knows about your condition and who can seek further help if necessary.

Make sure that you do not over exert yourself, have a hot bath or get hot, rather remain cool, as increasing the circulation can lead to more severe and rapid allergic reactions.  Concomitant asthma is an additional risk factor for food anaphylaxis.

All people who have had anaphylaxis should go to the local hospital Emergency Unit for further observation. This is because when the adrenaline wears off they may need further treatment, such as antihistamines, corticosteroids and occasionally oxygen and intravenous fluid therapy. If ever you go to hospital with anaphylaxis, a blood sample should be drawn to test for Tryptase (an anaphylaxis marker).  We would also recommend that you go to your local hospital and introduce yourself to the medical team so that they are aware of your anaphylactic tendency.  There is a risk of developing a delayed or biphasic reaction 8 to 12 hours after the initial reaction and you should remain in the Emergency Unit for at least 4 hours for observation of pulse, blood pressure and oxygen saturation (SATS).

Patients with Idiopathic Anaphylaxis may need to stay on low-dose oral cortisone for prolonged periods of time to prevent further attacks.

When referred to an allergy clinic, the specialist will take an extensive “allergy history” to try and clearly identify the cause of the anaphylaxis.  They will then perform allergy tests on a blood sample using the updated RAST test called a ImmunoCAP.  This test measures specific IgE antibodies in the patient’s blood to the suspected allergens.  Skin prick testing is not recommended for diagnosing anaphylaxis as these tests run the risk of triggering an allergic reaction.  Blood  RAST tests are completely safe and the recommended method of testing for the cause of anaphylaxis.  In some situations, patients may have what looks appears to be an anaphylactic reaction – which is in fact a fainting spell or sudden drop in blood pressure.  In this situation, we recommend a blood test which measures Mast Cell Tryptase a protein released in the body during anaphylaxis.  This is measured in the blood immediately and up to 6 hours after the reaction and can  confirm that anaphylaxis has indeed occurred. We can also measure Methyl Histamine in the urine as a marker of anaphylaxis having taken place.

Anaphylaxis emergency

Treatment and resuscitation of someone with suspected anaphylactic shock.

  • If the person is conscious and having breathing difficulties, help them to sit up.  If they are shocked with low blood pressure, they are better off lying flat with their legs raised.
  • If the person is unconscious, check their airways and breathing and lie them in the recovery position. Administer their asthma inhaler if available.
  • If you know that the person is susceptible to anaphylaxis, check if they carry a preloaded adrenaline syringe (EpiPen, Emerade or Jext). If necessary, help the person to inject it into the muscle of the outer thigh. This can be administered through clothing.
  • Dial 999 for an ambulance and tell the controller that you think the person may have anaphylactic shock. If available, antihistamine and steroid tablets should also be given.

How can we prevent anaphylaxis?

If you have ever had an anaphylactic reaction, you must be referred to an Allergy Clinic for full assessment so as to identify the cause of the reaction.

The following precautions need to be taken to prevent future anaphylactic reactions.

You must have at least two preloaded adrenaline auto-injectors called an Epipen, Emerade or Jext, carry one on your person and keep the other accessible at work or school.

  • Never forget to carry your Epipen or Jext pen with you at all times and make sure is within the expiry date.
  • Other people at home, work or college must be informed about your allergy, where you keep your Medication and how to use it
  • Make sure that your medication is always easily accessible and that the “expiry date” has not passed. Epipens and Emerade have a 12 month lifespan and Jext pen’s have a 24 months expiry date when new. Try not to expose them to direct sunlight.

You must also wear a prominent Medic Alert bracelet or necklace engraved with information for emergency medical workers to identify your specific anaphylactic condition.

Join the UK Anaphylaxis Campaign for information and support.  Their telephone Helpline is  01252 542029 and web address is

Don’t put yourself at risk

If you have one anaphylactic reaction there is no predicting how severe the next reaction might be. Future reactions may be identical, more severe or less severe but coexistent asthma is a risk factor for more severe reactions.   You are however unlikely to ever completely outgrow the allergy and need to take constant precautions.  Never assume that you will be all right just because you have not had a reaction for some time.

  • Check for ‘hidden’ ingredients on food labels and be very carefully with restaurant and “take away” food. Foods such as nuts and eggs may have a variety of different names and are often added to processed foods.
  • It is wise to seek the advice of an experienced dietician for help with excluding the offending allergen from your diet.

Certain non-allergic anaphylactoid conditions can mimic anaphylaxis

Systemic Mastocytosis is a rare condition where a person has a 20-fold increase in Mast Cells in the skin or internal organs – these cells can suddenly release enormous amounts of Histamine and Tryptase after minimal provocation. The result is a reaction that mimics a full blown anaphylactic reaction.  The serum Tryptase levels remain highly elevated even between attacks. Systemic Mastocytosis sufferers may have deeply freckled skin (Urticaria Pigmentosa) and associated flushing, headaches, urticaria, asthma, diarrhoea, liver enlargement and osteoporosis with bone pain (a bone biopsy is used in the diagnosis). Their skin typically develops urticarial lesions upon firm stroking – Darier’s sign.  This condition is commoner in children but thankfully many outgrow the disease by adolescence.

Hereditary Angioedema (HAE) is a familial condition that occurs in 1 in 80 000 people.  The typical symptoms begin in adolescence (often females).  The painful angioedema (tissue swelling) lasts for 2-3 days and involves the face, neck and limbs with debilitating non-rashing swelling and breathing difficulties. There is a deficiency or under-activity of the C1 Esterase Inhibitor enzyme and very low serum Complement C4 and CH50 on blood testing.

Carcinoid Syndrome can mimic anaphylaxis but there is usually considerable flushing, diarrhoea, wheezing and an associated  right heart murmur with high levels of Hydroxy Indole Acetic Acid (HIAA) in the urine.  Phaeochromocytoma is a benign adrenal tumour that secretes adrenalin-like chemicals that can mimic anaphylaxis with flushing but is associated with very high blood pressure.

Sometimes  Panic Attacks, Hysteria, Aspiration of foreign substances into the lungs, Epileptic seizures, Histamine-rich foods (Histamine Intolerance), Scromboid-Toxicity to spoilt fish, Lung clots (Pulmonary Embolism), Low blood sugar, Fainting spells and Heart Attacks  can closely resemble anaphylacxis and anaphylactoid reactions.


Visit the Anaphylaxis UK in the UK for more information.

The Royal College of Paediatricians also offer additional advice

Written and researched by Dr Adrian Morris
First published  June 2012 and last reviewed 4th January 2023.