Allergic reactions are an immune system response to a substance that the body perceives as harmful. Common synonyms for allergic reactions include hypersensitivity, sensitization, and intolerance. Anaphylaxis is a severe and sometimes life-threatening allergic reaction that can occur after exposure to an allergen. Other words that can be used to describe the symptoms of an allergic reaction include rash, swelling, hives, itching, and wheezing. Allergies can be triggered by a wide variety of substances, including foods, medications, insect stings, and environmental allergens like pollen and dust. Knowing the synonyms for allergic reactions can help individuals better understand and communicate their symptoms to healthcare providers.