Causes of anaphylaxis 

Anaphylaxis is caused by a problem with the immune system, which is the body's natural defence against illness and infection.

In the case of anaphylaxis, your immune system overreacts to a harmless substance and releases a number of different chemicals, such as histamine, to deal with the mistaken threat.


Some of the more common triggers for anaphylaxis are outlined below.

Insect stings

While any insect has the potential to trigger anaphylaxis, the vast majority of cases are either caused by bee or wasp stings.

It is estimated that around one in 100 people will experience an allergic reaction after a bee or wasp sting, but only a small minority of these people will go on to develop severe anaphylaxis.


Peanuts are the leading cause of food-related anaphylaxis, accounting for more than half of all cases.

Other foods known to trigger anaphylaxis include:

  • various types of nuts, such as walnuts, cashew nuts, almonds, brazil nuts and hazel nuts
  • milk
  • fish
  • seafood
  • eggs
  • some types of fruit, such as bananas, grapes and strawberries


Types of medication known to trigger anaphylaxis in a small amount of people include:

Most people sensitive to these types of medication will usually develop anaphylaxis as soon as they begin treatment, although this is not the case with NSAIDs.

The risks of these types of medication are very small, so in most cases the benefits of treatment outweigh the potential risk. For example, the risk of developing anaphylaxis:

  • after taking a NSAID-type painkiller – around one in 1,480
  • after taking penicillin – around one in 5,000
  • after being given a general anaesthetic – around one in 10,000

Contrast agents

Contrast agents are a group of special dyes used in certain medical tests to help certain areas of your body show up clearer on scans such as X-rays.

For example, a contrast agent injected into a blood vessel will help show up any problems in the vessel, such as a blockage, on the X-ray. This is known as an angiography.

The risk of developing anaphylaxis after being injected with a contrast agent is thought to be less than one in 10,000.

Idiopathic anaphylaxis

In as many as one in four cases of anaphylaxis, no trigger can be identified despite extensive testing.

This is known as idiopathic anaphylaxis.

Page last reviewed: 19/12/2012

Next review due: 19/12/2014