Antiphospholipid syndrome (APS) is caused by the body's immune system producing abnormal antibodies called antiphospholipid antibodies.

This increases the risk of blood clots developing in the blood vessels, which can lead to serious health problems, such as:

• DVT (deep vein thrombosis)
• stroke
• heart attack

It's not clear why these abnormal antibodies are produced, or why many people have antiphospholipid antibodies but don't develop blood clots.

A combination of genetic and environmental factors is thought to be responsible.

Antibodies are proteins produced by the immune system to help fight off infection and illness.

They're part of the body's defence system and are produced to help protect against "foreign invaders", such as bacteria and viruses.

Antibodies signal the immune system to release chemicals to kill these bacteria and viruses, and to prevent infection spreading.

In APS, the immune system produces abnormal antibodies that rather than attacking bacteria and viruses, mistakenly attack proteins found on the outside of cells in the blood and blood vessels.

It's not known how this causes the blood to clot more easily.

But most experts believe that keeping your blood at the correct consistency (not too runny and not too sticky) is a delicate balancing act that relies on different types of proteins and fats working together.

This balance may be disrupted by abnormal antibodies in people with APS.

Research into the genetics around APS is still at an early stage, but it seems the genes you inherit from your parents may play a role in the development of abnormal antiphospholipid antibodies.

APS isn't passed down directly from parents to children in the same way as other conditions, such as haemophilia and sickle cell anaemia.

But having a family member with antiphospholipid antibodies increases the chance of your immune system also producing them.

Studies have shown that some people with APS have a faulty gene that plays a role in other autoimmune conditions, such as lupus.

This may explain why some people develop APS alongside another immune system condition.

It's thought that one or more environmental triggers may be needed to start APS in some people.

Environmental factors that may be responsible include:

• viral infections, such as the cytomegalovirus (CMV) or slapped cheek syndrome (parvovirus B19)
• bacterial infections, such as E. coli (a bacteria often associated with food poisoning) or leptospirosis (an infection usually spread by certain animals)
• certain medications, such as anti-epileptic medicine or the oral contraceptive pill

Another theory is that many people with abnormal antiphospholipid antibodies only go on to develop APS if they have a higher risk of developing blood clots.

For example, if they:

• eat an unhealthy diet, leading to high cholesterol levels in the blood
• don't do enough exercise
• take the contraceptive pill or hormone replacement therapy (HRT)
• smoke
• are obese

But this doesn't explain why some children and adults who don't have any of these risk factors still develop APS.