Causes of antiphospholipid syndrome (APS) 

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 deep vein thrombosis (DVT), strokes or heart attacks.

However, many people have antiphospholipid antibodies and do not develop blood clots. It is not known why some people with these antibodies develop clots and others don't.

Antiphospholipid antibodies

Antibodies are proteins produced by the immune system to help fight off infection and illness. Antibodies are part of the body’s defence system produced to help protect against ‘invading forces’, such as bacteria and viruses. Antibodies can signal to the immune system to release chemicals to kill them and prevent infection spreading.

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

These abnormal antibodies are called antiphospholipid antibodies because they were originally thought to target fat molecules known as phospholipids. However, it's now thought that their real target is proteins attached to these fat molecules.

It is not known how the targeting of these proteins causes the blood to clot more easily. However, 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, and this balance may be disrupted by the abnormal antibodies in people with APS.

It is still not clear what causes the immune system to produce these abnormal antibodies or why most people with abnormal antibodies do not experience symptoms, although a combination of genetic and environmental factors is thought to be responsible.

Genetic factors

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

APS is not passed down directly from parents to children in the same way that other conditions are (such as haemophilia and sickle cell anaemia), but having a family member with antiphospholipid antibodies does increase the chance of your immune system also producing them.

Studies have shown that some cases of APS may be related to a similar mutated gene, known as the STAT4 gene.

This gene is also thought to play a role in other autoimmune conditions, such as lupus (a complex condition that affects many parts of the body and causes a wide range of symptoms).

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

Environmental factors

It is thought that one or more environmental triggers may be needed to trigger APS in some people.

Environmental factors that may be responsible include:

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:

However, this does not explain why some children and adults who do not have any of these risk factors still develop APS.

Page last reviewed: 06/11/2013

Next review due: 06/11/2015