Anaphylaxis refers to a generalized allergic reaction which is rapid in onset, and which may be severe. Although rare, some cases may lead to long term sequels or even cause death. It is estimated that between 3 and 30 in 100.000 persons per year will suffer an anaphylactic crisis, so you can see it is not very common.
By definition, at least two body systems should simultaneously be involved in this syndrome. The most common one is the skin in the form of itching, urticaria, generalized redness or angioedema; it is involved in 80% of cases. Other systems which may be involved are: the respiratory system (in the form of acute rhinitis, asthma or upper airway angioedema), the digestive track (with nausea, vomiting, stomach cramps or diarrhea), or the cardiovascular system (with palpitations, increased heart rate or fall in blood pressure). This may lead to dizziness, loss of consciousness, and in the worst scenario, to cardiac or respiratory arrest.
The most common causes are food in pediatric patients and drugs in adults. The third cause of anaphylaxis is bee and wasp stings, due to venom allergy.