Antiphospholipid syndrome (APS) is caused by the body’s immune system producing abnormal 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 women with these antibodies develop clots before having a miscarriage, while others do not (see below).
Antiphospholipid antibodies
Antibodies are proteins produced by the immune system to help fight off infection and illness. Antibodies identify threats, such as bacteria and viruses, and signal to the immune system to release chemicals to kill them and prevent infection spreading.
In APS, the immune system produces abnormal antiphospholipid antibodies which, rather than attacking bacteria and viruses, mistakenly attack some of the fats and proteins in your blood, causing it to clot.
It is not known why the antiphospholipid antibodies attack fats and proteins, which then causes the blood to clot. 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.
It is thought that when the antiphospholipid antibodies attack the fats and proteins, they disrupt the balance of the blood, making it stickier and increasing the risk of blood clots developing.
It is still not clear what causes the immune system to produce abnormal antibodies and why most people with abnormal antibodies do not experience symptoms. A combination of genetic and environmental reasons is thought to be behind it.
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.
A study that was carried out in 2008 found that almost half of people with APS had 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, and rheumatoid arthritis, which causes joint pain and stiffness.
This may explain why some people develop APS alongside another autoimmune 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 who do not have any of these risk factors develop APS.
