An allergy occurs when a person's immune system reacts to substances in the environment known as allergens. When a person who is allergic to a particular allergen comes into contact with it, an allergic reaction can occur.
Indoor and outdoor airborne allergens such as pollen, dust, and animal dander, can be breathed in and cause an allergic response. This can result in swelling (inflammation) of the nasal passages and that stuffy, blocked feeling. It can also result in other symptoms such as an itchy and runny nose, and sneezing.
Nasal allergies caused by seasonal allergens such as pollen are commonly known as 'hayfever' or seasonal allergic rhinitis. Nasal allergies caused by allergens that are in the environment all year round such as pet dander, are referred to as perennial allergic rhinitis.