Allergies happen when the immune system reacts to a foreign substance that gets inside the body. These substances are known as allergens. They include pollen, bee venom and pet dander. Allergies also can happen due to certain foods and medicines that don't cause reactions in most people. Allergies affect 1 in 5 Americans. Find info about allergy symptoms, causes, allergic reactions and treatments. Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. The mechanism of allergic reactions involves immunoglobulin E antibodies (IgE) binding to an allergen and then to a receptor on mast cells or basophils, where they trigger the release of inflammatory chemicals such as histamine. [16] Diagnosis is typically based on a person's medical history. [3] 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. An allergic reaction occurs when your body attacks a foreign, and typically harmless, substance. Learn about common allergens, allergy testing, and more. The meaning of ALLERGIC is of, relating to, affected with, or caused by allergy. How to use allergic in a sentence. Allergies are your immune system’s overreaction to foreign substances. 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. Allergic reactions happen when you come into contact with an allergen. Your immune system reacts by releasing histamines and other chemicals into your blood, causing symptoms that can irritate. 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.