Allergies occur when the immune system reacts to normally harmless substances called allergens. Common allergens include dust mites, pollen, foods, and some medications. Allergies are very common and can cause symptoms ranging from mild discomfort to life-threatening reactions. There are many different types of allergies including food, skin, dust, insect stings, pets, drugs, pollen, latex, mold, and cockroaches. Symptoms vary but can include sneezing, itchy eyes, rashes, hives, coughing, and difficulty breathing. Allergies are diagnosed through tests and treatments include avoidance of allergens and medications.