Allergic reactions occur when a person's immune system reacts to normally harmless substances in the environment.
Common causes of allergy symptoms include food allergies such as peanut allergy or milk allergy, and seasonal allergies resulting from grass, weed, tree pollen, or various molds. Cat allergies and dog allergies can also cause miserable symptoms such as itchy eyes, sneezing, nasal congestion, and wheezing. Allergic skin conditions can cause a rash and itchy skin.