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. Diagnosis involves the steps that your healthcare professional takes to find out whether you have an allergy. Your healthcare professional will likely: Ask detailed questions about your symptoms. Do a physical exam. Have you keep a detailed diary of symptoms and possible triggers. 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. Learn about different types of allergies, allergic reactions, their types, symptoms, and severity. 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] Allergies can cause a variety of symptoms such as a runny nose, sneezing, itching, rashes, swelling, or asthma. Allergies can range from minor to severe. Anaphylaxis is a severe reaction that can be life-threatening. Doctors use skin and blood tests to diagnose allergies. 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. Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Find out about allergies, including typical symptoms, common allergies, when to get emergency medical help and allergy testing and treatments. 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.