What is an allergic reaction? Allergic reactions are a set of symptoms that happen after you touch, inhale or eat something you’re allergic to. They can also happen after a bee sting. Examples of allergens that can set off allergic reactions include pollens, pets, dust, foods and medications. When you touch, inhale or eat something you’re allergic to, your body thinks it’s harmful and reacts to try to get rid of it. This can cause minor reactions like itching, sniffling and sneezing. But in some cases, allergic reactions can lead to life-threatening airway swelling and low blood pressure (anaphylaxis). Allergic reactions are sensitivities to substances called allergens that come into contact with the skin, nose, eyes, respiratory tract, and gastrointestinal tract. Allergic reactions can range from life-threatening to mildly uncomfortable. Here's how to recognize and respond to symptoms. An allergic reaction occurs when your body attacks a foreign, and typically harmless, substance. Learn about common allergens, allergy testing, and more. Allergic reactions can affect your eyes, skin, breathing or more. If you were having one, would you know it? WebMD tells you what you need to know. Allergic reactions can range from mild to life threatening. Learn how to treat common symptoms of allergic reactions, including rashes and anaphylaxis. Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Allergic reactions can range from mild to severe. Sometimes, allergies can trigger a life-threatening reaction known as anaphylaxis. Hay fever, also called allergic rhinitis, can cause: Sneezing. Itching of the skin, nose, eyes or roof of the mouth. Runny, stuffy nose. Tiredness, also called fatigue. These antibodies travel to cells that release chemicals, causing an allergic reaction. This reaction usually causes symptoms in the nose, lungs, throat, sinuses, ears, lining of the stomach or on the skin.