Understanding the Basics

An allergy is caused by an allergen which is a substance that can cause an allergic reaction. While food is one of the most common allergens, medicine, insect stings, latex and exercise can also cause a reaction.

Signs and Symptoms

An allergic reaction usually happens within minutes after being exposed to an allergen, but sometimes it can take place several hours after exposure. A reaction can involve any of these symptoms, and a person could have one or more of these symptoms regardless of the allergen:

  • Skin: hives, swelling, itching, warmth, redness, rash.
  • Breathing: coughing, wheezing, shortness of breath, chest pain/tightness, throat tightness, hoarse voice, nasal congestion or hay fever-like symptoms (runny itchy nose, watery eyes, and sneezing), trouble swallowing.
  • Stomach: nausea, pain/cramps, vomiting, diarrhea.
  • Heart and lungs: pale/blue colour, weak pulse, passing out, dizzy/lightheaded, shock.
  • Other: anxiety, feeling of “impending doom”, headache, uterine cramps, metallic taste.

The most dangerous symptoms of an allergic reaction are:

  • Trouble breathing caused by swelling of the airways (including a severe asthma attack for people who have asthma), and
  • A drop in blood pressure causing dizziness, light-headedness, feeling faint or weak, or passing out.

These are very serious if left untreated.


The term anaphylaxis is used to indicate the most extreme type of allergic reaction, usually affecting several body systems.  Anaphylaxis can involve any or all of the above signs and symptoms.  It can progress very quickly and may cause death without proper medical attention.

An anaphylactic reaction can occur within seconds of exposure and usually begins within a short time of the exposure to the allergen, but can be delayed up to two hours.

Important things to keep in mind

  • Do not ignore early symptoms, especially if you have had a reaction in the past. Always take a possible reaction seriously and act quickly.
  • Not every reaction will always look the same; a person can have different symptoms each time.
  • Anaphylaxis can occur without skin symptoms or hives.

Anaphylaxis Emergency Plan

Everyone at risk for anaphylaxis should have an anaphylaxis emergency plan with clear instructions on how to treat symptoms and strategies to reduce risks.

Source:  Some of the above information is taken from Anaphylaxis in Schools & Other Settings. Copyright © 2005-2011 Canadian Society of Allergy and Clinical Immunology

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Last updated: 2019-08-07