Anaphylaxis is a life-threatening allergic reaction to foreign protein antigens such as food, drugs, vaccines and bee stings. It is a very rare complication of immunisation (0.4-2 per million doses). Most episodes begin within 30 minutes of vaccination. 
Anaphylaxis occurs when an allergen reacts with specific IgE antibodies on mast cells and basophils (type 1 hypersensitivity reaction), triggering rapid release of stored histamine and rapid synthesis of inflammatory mediators. These cause capillary leakage, mucosal oedema and ultimately shock and asphyxia. It can vary in severity and rate of progression with manifestations over a few minutes or may be delayed by a few hours, adding to diagnostic difficulty. Once anaphylaxis is believed likely, immediate administration of epinephrine should occur. 
Anaphylaxis is a life-threatening allergic reaction. Common causes include foods such as peanuts, tree nuts (e.g. cashews, brazils, walnuts or almonds), sesame seeds, fish, shellfish, dairy products and eggs. Non-food causes include wasp or bee stings, natural latex (rubber), penicillin or any other drug or injection.
In some people, exercise can trigger a severe reaction – either on its own or in combination with other factors such as food or drugs (e.g. painkillers).