Allergies result when the body's immune system overreacts to a substance that is normally harmless. The substance, which is referred to as an allergen, could be anything from the pollen of a specific tree to a particular ingredient in a medication. It could be a puppy's dander or a food such as peanuts. While not everyone experiences allergies, some people are allergic to one specific thing, while others suffer a wide variety of different reactions to different allergens.

Two of the most common types of allergies are seasonal outdoor allergies (seasonal allergic rhinitis) and year-round indoor allergies (perennial allergic rhinitis). Those who sneeze and sniffle at certain times of the year can blame pollen for their symptoms. Tree pollen flourishes in the spring, while grass and weed pollen takes over in late summer and early autumn. Indoor allergens like dust, mould, mildew, and animal dander can irritate eyes, noses, and throats all year round.