Angioedema is often caused by a problem with the immune system, although there are many cases where no cause can be identified.

Allergic angioedema

Normally, your immune system protects your body from illness and infection by attacking germs in your body.

In allergic angioedema, your immune system mistakenly attacks harmless substances in your blood. Your body produces the chemical histamine, which causes the blood vessels in the area to expand, leading to swelling of the skin.

Substances known to trigger allergic angioedema include:

• certain types of food – particularly nuts, shellfish, milk and eggs
• some types of medication – such as penicillin, aspirin and non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen
• insect bites and stings – particularly wasp and bee stings
• latex – a naturally occurring rubber used to make medical gloves, balloons and condoms

Idiopathic angioedema

Cases of angioedema without an identifiable cause are known as idiopathic angioedema. It may be that a problem with the immune system sometimes causes it to "misfire".

In cases of idiopathic angioedema, certain triggers may lead to swelling, such as:

• anxiety or stress
• minor infections
• hot or cold temperatures
• exercise

Avoiding these triggers whenever possible may improve your symptoms.

Drug-induced angioedema

Some medications can cause angioedema. Angiotensin-converting enzyme (ACE) inhibitors, which are used to treat high blood pressure (hypertension), are usually responsible.

It's estimated that around 1-5% of people taking ACE inhibitors will develop drug-induced angioedema. Black people being treated with ACE inhibitors are three to four times more likely to develop this side effect than white people.

Around one in four cases of drug-induced angioedema occurs during the first month of taking an ACE inhibitor. The remaining cases develop many months or even years after treatment begins.

While they are useful in lowering blood pressure, ACE inhibitors can sometimes disrupt the "chemical balance" and trigger an episode of severe swelling.

Less common causes of drug-induced angioedema include:

• bupropion – a medication that helps people to give up smoking
• vaccines
• selective serotonin reuptake inhibitors (SSRIs) – a type of antidepressant
• COX-II inhibitors – a type of painkiller used to treat chronic pain conditions such as arthritis
• NSAIDs – a type of painkiller, such as aspirin or ibuprofen
• angiotensin-2 receptor antagonists – another medication used to treat high blood pressure
• statins – used to treat people with high cholesterol levels
• proton pump inhibitors (PPIs) – used to treat stomach ulcers

Hereditary angioedema

Hereditary angioedema is caused by a genetic mutation (a change in the DNA) in the C1 esterase inhibitor (C1-inh) gene. Genes are single units of genetic material that code for characteristics such as eye and hair colour.

As a result of the mutated C1-inh gene, the body does not make enough C1-inh protein. This protein plays an important role in regulating the immune system. Without enough C1-inh protein, the immune system can misfire and trigger the symptoms of angioedema.

Certain things are thought to trigger hereditary angioedema, such as:

• trauma – including surgery or infection
• the oral contraceptive pill
• pregnancy

The mutated C1-inh gene is passed down through families. If you have hereditary angioedema, you have a 50% chance of passing it on to your children.