Anaphylaxis: What It Really Is — and What You Should Do After an Episode

The word anaphylaxis was coined in 1902 by the French physiologist Charles Richet, who later won the Nobel Prize for the discovery. He and his colleague Paul Portier had been studying the effect of sea anemone venom on dogs, expecting to find that repeated exposure would produce immunity. It did the opposite. The immune system reacted with extreme, sometimes fatal force. Richet named the phenomenon anaphylaxis — from the Greek for "against protection."

More than a century later, anaphylaxis is still widely misunderstood — even by people who have experienced it.

What anaphylaxis actually is

Anaphylaxis is a severe, life-threatening, generalised or systemic hypersensitivity reaction. That phrase matters because it is quite specific. Anaphylaxis is not simply a very bad allergic reaction. It is a reaction that involves rapidly developing, potentially fatal problems with at least one — and often more — of the following:

• Airway — swelling of the throat or voice box (pharyngeal or laryngeal oedema)

• Breathing — spasm and tightening of the airways (bronchospasm), causing rapid, difficult breathing

• Circulation — dangerously low blood pressure (hypotension) or a racing heart (tachycardia)
  

In most cases there are also skin and mucosal changes — hives, flushing, swelling — but these do not have to be present. Critically, skin symptoms alone, however severe, do not constitute anaphylaxis. A patient covered in hives but breathing normally with stable blood pressure may be having an urticarial episode, which is serious and needs treatment but is not the same thing as anaphylaxis. The distinction matters because anaphylaxis requires a different, more urgent response. For the full clinical definition and guidance on assessment, see NICE Guideline NG258: Anaphylaxis assessment and referral after emergency treatment.

In most cases anaphylaxis works through an IgE-mediated mechanism. An allergen — say, peanut protein or penicillin — binds to IgE antibodies on the surface of mast cells, triggering a sudden, massive release of histamine, tryptase and other inflammatory chemicals. Blood vessels dilate, vascular permeability increases, blood pressure drops, and airways constrict — all within minutes.

The scope of the problem

Hospital admissions in the UK for anaphylaxis from all causes reached 4,323 in 2023/24 — an increase of 17.8% from the year before, and more than double the figure recorded in 2002/03 (NHS Hospital Episode Statistics, 2023/24, via Allergy UK). Between 1998 and 2018, 101,891 people were admitted to hospital for anaphylaxis in the UK, of whom around 30% had a food trigger. Food anaphylaxis admissions increased at an annual rate of 5.7% over that period (Baseggio Conrado et al., BMJ 2021) — with the steepest increase in children under 15.

Despite this rise, the picture is not entirely bleak. Over the same 20-year period, the fatality rate from food anaphylaxis more than halved — from 0.7% in 1998 to 0.3% in 2018 — suggesting that better awareness and faster treatment are saving lives. In absolute terms, approximately 10 people die in the UK each year from food-induced anaphylaxis — a figure that, while devastating in individual terms, reflects how survivable anaphylaxis is when treated promptly and correctly.

What triggers it

The most common causes of anaphylaxis vary with age. In children, food allergy is the predominant trigger — particularly peanuts, tree nuts, milk and egg. In adults and older people, medicinal products become a much more common cause, alongside insect venom. In the UK, it is estimated that 500,000 people have had a venom-induced anaphylactic reaction, and 220,000 people up to the age of 44 have had a nut-induced anaphylactic reaction.

Other established triggers include latex, exercise (sometimes in combination with a food), and contrast media used in medical imaging. A proportion of anaphylaxis is idiopathic — meaning no trigger can be identified even after thorough investigation.

How it is diagnosed — and why serum tryptase matters

One of the more common sources of confusion around anaphylaxis is how it is confirmed. In the acute setting, the diagnosis is clinical — a clinician assesses the symptoms and responds accordingly. But a blood test called serum tryptase can provide crucial supporting evidence.

Tryptase is an enzyme released by mast cells during anaphylaxis. The rise in tryptase levels starts to be detected in serum within minutes of anaphylaxis, gradually reverting to normal over the next 6 to 24 hours depending on the height of the increase — and the level often correlates with the severity of the reaction. An acutely elevated serum tryptase is significant for two reasons: it helps confirm that what happened was indeed anaphylaxis (and not a panic attack, a cardiac event, or severe asthma — all of which can look similar), and it indicates that the reaction was potentially life-threatening, which shapes the follow-up needed.

A baseline tryptase level, measured when the person is well, is also useful. If the acute level is significantly elevated above baseline, this strongly supports a diagnosis of anaphylaxis. In some patients, a persistently elevated baseline tryptase may indicate an underlying mast cell disorder, which warrants further specialist investigation.

The biphasic reaction — a lesser-known risk

Here is something many patients are not told after an episode: anaphylaxis can come back.

A biphasic reaction is a recurrence of anaphylaxis within 72 hours of apparent full recovery, without any further exposure to the triggering allergen. The mechanisms are not fully understood, but it is enough of a risk that NICE guidance recommends that adults and young people aged 16 and over who have had emergency treatment for suspected anaphylaxis should be observed in hospital for a minimum of 6 to 12 hours from the onset of symptoms, depending on how quickly and easily the reaction was controlled. Children under 16 should be admitted under the care of a paediatric medical team.

Adrenaline: the only first-line treatment

Adrenaline (epinephrine) given intramuscularly is the first-line treatment for anaphylaxis. Antihistamines and steroids may be administered as well, often intravenously, but adrenaline is the key component needed to quickly reverse the life-threatening elements of anaphylaxis.

Following emergency treatment, NICE guidance recommends that every person should be provided with two adrenaline auto-injectors before discharge, along with clear instructions on when and how to use them. Despite this, less than 58% of people with previous food anaphylaxis have a prescription for an adrenaline auto-injector — a significant gap between guidance and practice.

Why specialist follow-up matters — and why it often doesn't happen

NICE guideline NG258 is unambiguous: after any episode of suspected anaphylaxis, patients should be offered a referral to a specialist allergy service for a full assessment. The purpose of this referral is to confirm the diagnosis, identify the trigger, and provide a personalised management plan — including prescribing adrenaline auto-injectors, advising on allergen avoidance, and discussing whether desensitisation (immunotherapy) is appropriate. In practice, this does not always happen. Paediatric allergy assessment following anaphylaxis in children is available with our consultant paediatricians Dr Emily Derrick and Dr Tammy Rothenberg, and for adults with Dr Gururaj Arumugakani.

On the NHS, waits for specialist allergy services can be lengthy. A private allergy consultation can provide faster access to the investigation and management that NICE recommends.

After an episode of anaphylaxis: what a specialist can do

One practical question that comes up often after an anaphylactic episode is when allergy testing can be arranged. The answer matters because testing too soon carries a specific risk.

In the weeks following anaphylaxis, the mast cells — the immune cells whose sudden, massive activation drives the reaction — can be temporarily depleted of their mediators.

This is sometimes called the refractory period. Because allergy skin tests work by triggering a local mast cell response, testing during this window may produce a falsely negative result: the test appears to show no allergy, not because the allergy is absent but because the mast cells cannot mount a normal response.

Current guidance recommends waiting at least four weeks after a major allergic reaction before allergy testing. A positive result during this period can be relied upon — if the mast cells react despite possible depletion, the allergy is confirmed. A negative result cannot be relied upon and should not be used to conclude that the allergy is absent. This is why timing matters when arranging your follow-up.

It is also worth noting that this is precisely why the four-week wait is standard advice before both skin prick tests and specific IgE blood tests following a significant allergic reaction — as we explain in more detail in our guide to preparing for an allergy test.

A specialist allergy consultation following a suspected anaphylactic episode involves a detailed review of the clinical history — what happened, when, in what circumstances, and what was done — alongside targeted investigation to identify the likely trigger. This might include:

• Specific IgE blood testing for suspected food, venom or drug triggers

• A comprehensive allergen panel if the trigger is unclear (ALEX3)

• Skin prick testing where appropriate

• Penicillin challenge for suspected drug allergy, to confirm or rule out the diagnosis

• Assessment of baseline serum tryptase
  

For insect venom allergy specifically, venom immunotherapy — a structured desensitisation programme — can significantly reduce the severity of future reactions and, in some patients, eliminate the allergy entirely.

The outcome of a specialist consultation is a written management plan, prescriptions for adrenaline auto-injectors with training on how to use them, a letter to the patient's GP, and a clear understanding of what to avoid and what to do if another reaction occurs.

Supporting resources

Anaphylaxis UK is the leading UK charity for people living with anaphylaxis and severe allergies. They provide a free allergy alert register and practical guidance on emergency management.

If you or your child has experienced a suspected anaphylactic episode, registering for allergy alerts is a straightforward precaution. Their emergency guidance — what to do in an anaphylactic emergency — is clear and practical.

Book a consultation → or contact us by phone or WhatsApp, Mon–Sat 9am–7pm: 0207 637 9711

Dr Gururaj Arumugakani, our Consultant Allergist and Clinical Immunologist at The London Allergy Clinic, sees adults and adolescents aged 12 and over with suspected anaphylaxis and its triggers — including food allergy, drug allergy and insect venom allergy — at our clinics in Marylebone, Fitzrovia and High Barnet. No GP referral is required. You can read what our patients say on our patient reviews page.

Sources

Nobel Prize Foundation. Charles Richet — Nobel Prize in Physiology or Medicine 1913. nobelprize.org

National Institute for Health and Care Excellence. Anaphylaxis: assessment and referral after emergency treatment. NICE Guideline NG258 (2023). nice.org.uk/guidance/ng258

Shaker MS et al. Anaphylaxis: A 2023 practice parameter update. Annals of Allergy, Asthma & Immunology 2023; 131(2):124-176. annallergy.org

Lafuente A et al. Caution against temporary tolerance and negative skin tests after anaphylaxis. Journal of Investigational Allergology and Clinical Immunology 2022; 32(2). jiaci.org

NHS. Anaphylaxis. nhs.uk/conditions/anaphylaxis

Allergy UK. Statistics and figures. allergyuk.org

Baseggio Conrado A et al. Food anaphylaxis in the United Kingdom: analysis of national data, 1998–2018. BMJ 2021; 372:n251. ncbi.nlm.nih.gov/pmc/articles/PMC7885259

Imperial College London. Deaths from food allergy rare and decreasing in the UK, finds study. February 2021. imperial.ac.uk

Anaphylaxis UK. About anaphylaxis. anaphylaxis.org.uk

National Center for Biotechnology Information. Measuring serum tryptase after suspected anaphylaxis. In: Drug Allergy. NCBI Bookshelf. ncbi.nlm.nih.gov/books/NBK274147

Resuscitation Council UK. Treatment of anaphylaxis. resus.org.uk