An allergic reaction occurs when the immune system becomes hypersensitive to foods, pollen, medications, bee venom, and other substances. Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Allergic reactions can cause swelling, hives, rashes, and anaphylaxis in severe cases. Learn about allergy testing, symptoms, causes, treatment, and prevention. Many people suspect they have allergies. Learn about allergen types, common allergy signs and symptoms, how to get diagnosed and available treatment options. 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. 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 affect your eyes, skin, breathing or more. If you were having one, would you know it? WebMD tells you what you need to know. Allergy symptoms can range from mild to severe, even life-threatening. Find out what treatments and prevention tips can help. For millions of people around the world, this isn’t just a passing annoyance; it’s a sign of an allergic reaction. Allergies are among the most common chronic conditions of modern times, affecting people of all ages and backgrounds. An allergic reaction happens when the immune system overreacts to certain substances, causing symptoms like hives or difficulty breathing. Severe allergic reactions, called anaphylaxis, can be life-threatening and require immediate emergency care.