Anaphylaxis (also known as an anaphylactic reaction) is a serious allergic reaction that is rapid in onset and on rare occasions causes death. There may often be a rash or swelling affecting the skin and sometimes vomiting and diarrhoea. However, the more serious and severe features of the reaction are caused by swelling and tightening of the airways causing difficulty in breathing and/or a sudden fall in the blood pressure leading to dizziness and even collapse.