Anaphylaxis is a severe allergic reaction that results from the interaction of an allergen (see causes below) with specific immunoglobulin E (IgE) antibodies bound to mast cells in the skin and lungs and basophils in the blood. The mast cell releases chemicals that act on blood vessels to produce a wide range of clinical effects throughout the body.
Anaphylactoid reactions are clinically similar to those experienced in anaphylaxis. The only difference is that anaphylactoid reactions are non-IgE mediated and may be less severe.
Severity of reactions from anaphylaxis can vary from mild symptoms to sudden death. In any case, medical attention should be sought immediately and appropriate treatment given.
What causes anaphylaxis?
A wide variety of substances can cause anaphylaxis or anaphylactoid reactions. Approximately one third of all cases have no known cause (idiopathic).
What are the signs and symptoms of anaphylaxis?
Food-induced anaphylaxis often produces skin reactions and respiratory symptoms whilst drug- or venom-induced anaphylaxis more often produces shock. Symptoms usually occur within 5-60 minutes of contact with the allergen, but sometimes occur after several hours, or even 3-4 days later. Fast onset and rapid progression of symptoms usually indicates severe, life-threatening anaphylaxis. One or more organ systems may be involved. Typical features include:
Diagnosis of anaphylaxis
Because acute anaphylaxis can be immediately life threatening, diagnosis must be made quickly and efficiently, often while administering initial medication. Diagnosis is essentially made on the basis of:
- Typical symptoms and signs, involving at least two organ systems
- Development of specific symptoms after exposure to a known allergen
- Exclusion of other diseases that may have similar signs and symptoms
What is the treatment of anaphylaxis?
Acute anaphylaxis must be treated as a medical emergency with stabilisation of airway, breathing and circulation. Intramuscular adrenaline (epinephrine) must be given immediately to patients with signs of shock, airway swelling, or definite difficulty in breathing. This is followed by treatment with an antihistamine, corticosteroid and perhaps other drugs.
Adrenaline may not be necessary for skin manifestations of anaphylaxis. Treatment with antihistamines may be all that is required.
Prevention is the best medicine. All those at risk of anaphylaxis should wear a Medic Alert/emergency bracelet with full details of allergies and contact details of their doctor. In some cases, a patient or caregiver should always carry an emergency kit containing self-injectable adrenaline and antihistamine tablets.
Adrenaline is available as an auto-injector, EpiPen® (0.3 mg) and EpiPen® Jr (0.15 mg). They are prescribed for emergency use in people with a history of an anaphylactic reaction. If they are used, the patient should also obtain immediate medical care. The adrenaline may cause a fast or irregular heart beat, nausea and breathing difficulties.
- The EpiPen needs to be stored away from heat and light.
- Ask your specialist to explain when you should use it.
- Learn how to use it, and practise regularly with an EpiPen trainer. Instructions are on the EpiPen website.
- Form a fist around the EpiPen and pull off grey cap. Place black end against outer mid thigh. Push down hard until a click is heard or felt and hold in place for 10 seconds. Remove the pen. Massage the injection site for a few seconds.
- Book: Textbook of Dermatology. Ed Rook A, Wilkinson DS, Ebling FJB, Champion RH, Burton JL. Fourth edition. Blackwell Scientific Publications.
On DermNet NZ:
- Anaphylaxis – emedicine, the online textbook
- Anaphylactic Shock – emedicinehealth
- Anaphylaxis Campaign, UK
- Anaphylaxis Canada
- Allergy New Zealand
- Anaphylaxis Australia
- Food Allergy and Anaphylaxis Alliance
- Food Allergy and Anaphylaxis Network, USA
- Australasian Society of Clinical Immunology and Allergy including anaphylaxis resources
- Allergy Action
See the DermNet NZ bookstore