What is an allergic reaction? An allergic reaction is an inflammatory process that is triggered by a foreign substance known as an allergen. Allergens are usually proteins and individual susceptibility varies markedly from person to person. Common allergens include animal dander from cats or dogs, poison ivy (rhus), bee sting venom, airborne molecules such as ragweed pollen, and various types of drugs such as penicillin and sulfa antibiotics. Allergic reactions typically develop after repeated exposure to an allergen. Symptoms of allergic reactions include skin lesions known as hives or urticaria, tongue or facial swelling, sneezing, itchy eyes, nausea, vomiting, and a variety of rashes. Allergic reactions may range from mild, self-limited sniffles to life-threatening conditions such as anaphylaxis and angioedema, in which the mouth and tongue may swell to a point that makes it difficult to breathe. In most cases, physicians diagnose allergic reactions based on clinical signs and symptoms. This clinical diagnosis may be followed up with a series of blood tests or skin tests to establish the identity of the particular allergen. Allergic reactions are treated with medications. Antihistamines may be used to treat mild allergic reactions. As allergic reactions become more severe, doctors typically prescribe steroids in addition to antihistamines. For life-threatening allergic reactions, injected epinephrine may be added. The definitive treatment for allergic reactions is to avoid the offending allergen entirely or to undergo desensitization therapy under the care of an allergist. Seek immediate medical care (call 911) for serious symptoms of an allergic reaction, such as chest pain, chest tightness, difficulty breathing, nausea, rapid heartbeat, swelling in the throat or mouth, dizziness or unconsciousness, flushing with hives, or wheezing. Seek prompt medical care if you have any but the mildest symptoms of an allergic reaction, or if you are being treated for an allergic reaction and you fail to improve or have concerns about your condition.