Allergies are caused by an abnormal immune system response to usually harmless substances called allergens. Common allergens include pollen, mold, dust, foods, insect stings, and animal dander. Allergic reactions occur when a person is exposed to an allergen, causing the immune system to produce antibodies that trigger the release of histamine and other chemicals from mast cells, resulting in symptoms like sneezing, itchy eyes, rashes, and more severe reactions like anaphylaxis in some cases. Allergies can be diagnosed through skin or blood tests to identify specific allergens, and are treated with medications that reduce symptoms or immunotherapy to desensitize the immune system.