What is an allergy?

An allergy is a multifactorial disease with both environmental and hereditary factors. It occurs when the immune system responds to allergens (environmental substances), resulting in an allergic reaction.

In response to allergens, the immune system produces an antibody called immunoglobulin E (IgE), releasing chemicals like histamine into the bloodstream. Histamine helps rid the body of allergens, causing symptoms such as sneezing, itching, hives and watery eyes.

Allergic rhinitis (hay fever) is a collection of symptoms, predominantly in the nose and eyes, to allergens such as dust, animal dander and pollen. Allergic rhinitis or “hay fever” is one of the most common types of allergy, causing an inflammation of the tissues in the nose, often accompanied by conjunctivitis (red, itchy, and/or watery eyes).

Allergies can impact your daily activities and make you feel easily frustrated and tired, but most cases are mild.


Allergies are triggered by a reaction to allergens. Most allergy cases are seasonal, but some people experience symptoms year-round.

Common allergens include:

Pollen: A fine powdery substance produced by species of trees, grass and weeds. In Canada, tree pollen peaks in the spring (late April to May), while grass and weed pollen peak in the summer (late May to mid-July) and fall (mid-August to October).

Mould: A type of fungi caused by excessive water and humidity. They’re typically found in poorly ventilated areas like bathrooms and basements.

Pet dander: Flecks of dead skin shed by pets with fur or feathers like dogs, cats and birds. Dander floats in the air of your home and is the main source of pet allergies.

Dust mites: Tiny, microscopic bugs that live in your home. They can collect on humid, dust-prone furnishings like pillows, mattresses, carpets and stuffed toys.

An allergy skin test is the most effective method to confirm underlying allergies. With skin testing, specific allergens can be identified through a skin prick test by using small amount of the suspected allergen causing the allergy by scraping the skin – forearm, upper arm or back – with these allergens. The skin is then watched for any signs of reaction, usually swelling and redness of the site. Several allergens can be tested at the same time.


Allergy symptoms vary from person-to-person. They can either develop quickly within a few minutes or over the course of a few hours. 

Common symptoms include:

  • Runny nose
  • Sneezing
  • Itchy nose, eyes and roof of mouth
  • Red and watery eyes
  • Congestion that can lead to a headache
  • Pressure in the nose and cheeks
  • Ear fullness and popping
  • Post nasal drip

Related symptoms

Allergic Conjunctivitis: Inflammation of the tissue lining the conjunctiva (eyelids) due to an allergic reaction. When your eyes are exposed to allergens, histamine is released and the blood vessels in the conjunctiva become swollen. This causes your eyes to redden, itch and tear.

Sinusitis: Inflammation of the air cavities within the passages of the nose. Sinusitis occurs when environmental pollens irritate or infect the nasal passages. Symptoms of sinusitis include pain, congestion and headaches.

Are allergies and asthma related?

Allergies and asthma are related conditions linked by a common airway. They both affect our breathing by blocking the free passage of air between the nose and the lungs.

With allergies, the obstruction occurs in the upper area of the airway when the layers of the nose become inflamed. Sneezing helps to clear the mucus from the upper part of the airway.

With asthma, the breathlessness and wheezing are caused by a narrowing of the bronchioles. Inflammation of the small airways’ layers may cause mucus to increase, making the obstruction worse. The dry cough that develops in the airways helps to clear out the mucus.
An allergic reaction in the lungs can develop into asthma, so it’s important to immediately address the symptoms to control both conditions.

Did You Know:

  • Respiratory allergies such as allergic rhinitis affect 1 in 5 Canadians
  • 80% of people with asthma also suffer from allergic rhinitis or sinusitis
  • Allergies can affect your quality of life at work, school, and play
  • Allergic rhinitis and sinusitis are associated with more severe or frequent asthma symptoms
  • 75% of asthma patients also have seasonal allergies.
  • The two conditions frequently overlap, as several of the same allergens are known to trigger asthma and seasonal allergy exacerbations.
  • Seasonal allergies tend to make asthma worse and may be a risk factor for the development of asthma.
  • Allergies play a key role in asthma, asthma control, and management
  • Effective treatment of seasonal allergies can reduce asthma symptoms and may even help prevent the development of asthma.

Managing Allergies

Immunotherapy Decision Making Tool

Allergy Treatments

Breathe Easy: Allergies

Download a copy of our Breathe Easy Booklet Series on allergies and asthma. It provides useful information for understanding and managing your allergies alongside your asthma. [Click here to download in French].