The immune system defends against foreign substances, with antigens triggering an immune response. In some cases, excessive immune responses cause inflammation and tissue damage, known as an allergic response. Allergens are antigens causing allergies, and there are four types of hypersensitivity. Type one, or immediate hypersensitivity, is the most common, involving allergens like pollen. Upon re-exposure, immunoglobulin E antibodies trigger mast cells and basophils to release inflammatory substances, potentially causing severe reactions like anaphylaxis. Treatments include antihistamines for mild symptoms, epinephrine for anaphylaxis, and, in some cases, desensitization for unavoidable allergens.