Allergies occur when your immune system reacts to a harmless substance as though it is a threat. This substance, known as an allergen, can be something you inhale, ingest, or come into contact with. For most people, these substances are harmless, but your immune system may respond as if they are dangerous.
Allergies can be seasonal or persistent throughout the year. The symptoms you experience depend on the type of allergen and the severity of your reaction.