What are allergies?Updated 7 months ago

Allergies are an overreaction of the immune system. People who have allergies have a hyper-alert immune system that overreacts to a substance in the environment called an allergen. Exposure to what is normally a harmless substance, such as pollen, causes the immune system to react as if the substance is harmful.

Allergies are a very common problem, affecting at least 2 out of every 10 Americans.

Additional information: Allergies? Understanding the Immune System