APS is an autoimmune condition. This means the immune system, which usually protects the body from infection and illness, attacks healthy tissue by mistake.
In APS, the immune system produces abnormal antibodies called antiphospholipid antibodies.
These target proteins attached to fat molecules (phospholipids), which makes the blood more likely to clot.
It's not known what causes the immune system to produce abnormal antibodies.
As with other autoimmune conditions, genetic, hormonal and environmental factors are thought to play a part.
APS can affect people of all ages, including children and babies.
But most people are diagnosed with APS between 20 and 50 years of age, and it affects 3 to 5 times as many women as men.
It's not clear how many people in the UK have the condition.
Diagnosing APS can sometimes be tricky, as some of the symptoms are similar to multiple sclerosis.
This means blood tests to identify the antibodies responsible for APS are essential in diagnosing the condition.
Although there's no cure for APS, the risk of developing blood clots can be greatly reduced if it's correctly diagnosed.
These reduce the likelihood of unnecessary blood clots forming, but still allow clots to form when you cut yourself.
Treatment with these medications can also improve a pregnant woman's chance of having a successful pregnancy.
With treatment, it's estimated there's about an 80% chance of having a successful pregnancy.
Most people respond well to treatment and can lead normal, healthy lives.
But a small number of people with APS continue to experience blood clots despite extensive treatment.
In very rare cases, blood clots can suddenly form throughout the body, resulting in multiple organ failure. This is known as catastrophic antiphospholipid syndrome (CAPS).
CAPS requires immediate emergency treatment in hospital with high-dose anticoagulants.