Allergies occur when the immune system reacts to a substance, known as an allergen, that is usually harmless. Common allergens include pollen, dust mites, pet dander, and certain foods like peanuts or shellfish. When a person with allergies comes into contact with these substances, their body may produce symptoms such as sneezing, itching, or swelling. The severity of allergic reactions can vary from mild to severe. In some cases, exposure to an allergen can lead to anaphylaxis, a life-threatening condition that requires immediate medical attention. Treatment options for allergies include avoiding allergens, taking medications like antihistamines, and in some cases, undergoing allergy shots to build tolerance.