For most people, allergy symptoms are mild or moderate, but for some hyperallergic people, an allergic reaction can be life-threatening. Anaphylaxis is a runaway allergic reaction that involves the whole body. Anaphylaxis can cause extreme swelling in the lungs and airways that can make breathing or swallowing difficult, as well as severe abdominal pain, cramps, vomiting, diarrhea, and mental confusion or dizziness. Anaphylaxis is a medical emergency requiring immediate treatment.