Allergies are an overreaction of the body's immune system, which starts fighting substances that are usually harmless (such as dust mites, pollen, or a medicine). This overreaction can cause a rash, itchy eyes, a runny nose, trouble breathing, nausea, and diarrhea. Most allergic reactions are mild and can be treated at home but severe allergic reactions (anaphylaxis) can be life-threatening.