Signs and symptoms

In this section

What is anaphylaxis?

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

Watch Professor John Warner OBE, a member of our Clinical & Scientific Panel, explains anaphylaxis and allergy:

What can cause anaphylaxis?

Common causes include foods such as peanuts, tree nuts (e.g. almonds, walnuts, cashews, and Brazil nuts), sesame, fish, shellfish, dairy products and eggs.

Non-food causes include wasp or bee stings, natural latex (rubber), 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).

What are the symptoms of a severe allergic reaction?

  • generalised flushing of the skin
  • nettle rash (hives) anywhere on the body
  • sense of impending doom
  • swelling of throat and mouth
  • difficulty in swallowing or speaking
  • alterations in heart rate
  • severe asthma
  • abdominal pain, nausea and vomiting
  • sudden feeling of weakness (drop in blood pressure)
  • collapse and unconsciousness

A patient would not necessarily experience all of these symptoms in the same episode.

Why does anaphylaxis occur?

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 that it wrongly perceives as a threat.

The symptoms are 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 interaction between an allergic antibody called Immunoglobulin E (IgE) and the substance (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 swelling. In people with asthma, the effect is mainly on the lungs. There may also be a fall in blood pressure.

What is the treatment for a severe reaction?

Adrenaline auto-injectors are prescribed 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. Our online training programme AllergyWise provides information on what to do in an emergency. More information on adrenaline auto-injectors.

View our tips on what to do in an emergency.

Find out more

A short film by the Royal College of Physicians was created to coincide with the launch of new guidelines on the management of adult anaphylaxis. Click here to watch the film and find out more about anaphylaxis and the work of the Anaphylaxis Campaign.

Who is at risk from anaphylaxis?

If a patient has suffered a bad allergic reaction in the past, whatever the cause, then any future reaction is also likely to be severe. If a significant reaction to a tiny dose occurs, or a reaction has occurred on skin contact, this might also be a sign that a larger dose may trigger a severe reaction. It is particularly important that those with asthma as well as allergies are seen by an allergy specialist because asthma can put a patient in a higher risk category. Anyone who believes they suffer from an allergy should see their GP. Where there is a chance of an allergy being severe, the GP should refer the patient to an NHS allergy clinic.