Understanding Anaphylaxis

Updated 28th November 2017, updated 21st October 2024

What is it?

Anaphylaxis is an extreme and severe allergic reaction. The whole body is affected, often within minutes of exposure to the allergen but sometimes after hours.

After an initial exposure or "sensitising dose" to an allergen (such as a bee sting) at some point in the person's life, the person's immune system becomes sensitised to that allergen. On a subsequent exposure or "shocking dose", an allergic reaction occurs.

In the United Kingdom, mortality rates for anaphylaxis have been reported as up to 0.05 per 100,000 population, or around 10-20 a year. In 2005 there were 12 deaths associated with severe reaction to foods.(1) Anaphylactic reactions requiring hospital treatment appear to be increasing, with authorities in the UK reporting a threefold increase between 1994 and 2004.(2)

More people are becoming more sensitive to more allergens.

Any allergic reaction, including the most extreme form, anaphylactic shock, occurs because the body's immune system reacts inappropriately in response to the presence of a substance - the allergen - that it wrongly perceives as a threat.

An anaphylactic reaction is caused by the sudden release of chemical substances, including histamine, from cells in the blood and tissues where they are stored. The release is triggered by the reaction between the allergic antibody Immunoglobin E (IgE) and the allergen causing the anaphylactic reaction.

This mechanism is so sensitive that minute quantities of the allergen can cause a reaction. The released chemicals act on blood vessels to cause the swelling in the mouth and airway and anywhere on the skin. There is a fall in blood pressure and, in asthmatics, the effect is mainly on the lungs.

What are the Triggers?

Common causes include

'Nuts' - peanuts, tree nuts (e.g. almonds, walnuts, cashews, Brazils), sesame seeds
Seafood and shellfish,
Dairy products and eggs.
Venom - wasp or bee stings, jellyfish stings
Non-food causes include natural latex (rubber) and penicillin (or any other drug or injection).

In some people, exercise can trigger a severe reaction, either on its own or in combination with other factors such as food or drugs (e.g. aspirin).
Commonly, most allergens are naturally occurring proteins.

What are the symptoms?

Anaphylaxis can present with many different symptoms due to the systemic effects of histamine release.
The most common areas affected include:

Skin (80% to 90%),

generalised flushing of the skin
nettle rash (hives) anywhere on the body

Respiratory (70%),

swelling of throat and mouth
difficulty in swallowing or speaking
severe asthma

Gastrointestinal (30% to 45%),

abdominal pain, nausea and vomiting

Heart and vasculature (10% to 45%),

alterations in heart rate
sudden feeling of weakness (drop in blood pressure)

Central nervous system (10% to 15%).

collapse and unconsciousness

Symptoms can develop within a few minutes or over several hours, furthermore, an individual may not necessarily experience all of these symptoms (3).

What is the treatment?

Pre-loaded adrenaline auto-injection pens are available on prescription for those believed to be at risk. Adrenaline (also known as epinephrine) acts quickly to constrict blood vessels, relax smooth muscles in the lungs to improve breathing, stimulate the heartbeat and help to stop swelling around the face and lips.

There is no contraindication to administering adrenalin via auto-injector to anyone who is displaying signs of an anaphylactic reaction.

Remote Protocols

In a remote environment where the casualty is far from help, the consequences of a severe or anaphylactic reaction can be life threatening due to the distance to definitive medical care. As such the treatment suggested below would neither be appropriate nor advocated for an urban environment.

Administer adrenaline via auto-injector every 5 minutes until improvement occurs.
If the patient is awake and can swallow give a double dose chlorphenamine ("Piriton") immediately and continue with the normal dose for two days, the reduce gradually. Chlorpnemaine takes around 20 minutes to take effect and will last 4-6 hours (4)

25 – 50 mg of diphenhydramine ("Benadryl") by mouth every 4-6 hours (6)

Both chlorphenamine and diphenhydramide are sedatives - do not allow the casualty to drive or operate machinery whilst taking these medications and until they are fully recovered.

If adrenaline is not available, phenylephrine or pseudoephedrine nasal spray or drops (8-10 sprays / drops per nostril) every 15 minutes until improvement occurs.(4)
For breathing difficulties, a salbutamol asthma inhaler may help.
Administer oxygen if available.
EVACUATE - an improvement may be followed by a relapse, in which case start the treatment again.

WARNING: Apart from adrenaline and oxygen, the above drugs are not licenced for the treatment of anaphylaxis. In a remote environment necessity will dictate; the above suggestions have been seen to be of benefit and relatively safe to use compared to the consequences of an untreated reaction but YOU take responsibility for your actions.

Further reading: Administering Medications