Definition: Anaphylaxis is a severe allergic reaction that can cause shortness of breath, swelling in the face, lips, tongue, or throat, hives, itching, wheezing, chest tightness, rapid heartbeat, sweating, nausea, vomiting, and difficulty breathing if you are allergic to certain foods. Detailed definition: Anaphylaxis is a serious allergic reaction that occurs when you react to an allergen (such as a tree pollen, food, animal protein, or smoke) in your body. The symptoms can vary, from minor skin reactions to life-threatening emergency situations.