An allergy sign is a physical reaction to an allergen, which can be anything from pollen to certain foods. One of the most common signs of an allergy is a sneeze, which is a reflexive action that expels air from the lungs and is usually accompanied by a distinctive sound. Sneezing is caused by irritation in the nose, throat, or sinuses, and can be triggered by allergens such as dust, pollen, pet dander, and certain foods.