What causes APS

As of yet, we simply don’t know why people develop APS, why some patients go on to have blood clots while others don’t, why some women (but not all) have pregnancy problems and why some patients are affected by symptoms more than others – vital research is needed before we can answer these questions.

However, we do know that the root cause is an over-activity of the immune system. Normally, the immune system protects the body against foreign invaders - such as viruses or bacteria - by producing antibodies that destroy these harmful substances. But sometimes the body gets confused and, in a case of mistaken identity, it makes antibodies against itself causing an autoimmune disorder.

In patients with APS, the body produces harmful antibodies called antiphospholipid antibodies (aPL). These are abnormal antibodies which attack proteins that are linked to fats in the body.

The most important of these proteins is called beta-2-glycoprotein1. When aPL stick to this protein it forms a structure called aPL-beta2-glycoprotein1. This structure becomes attached to fats called phospholipids, a type of phosphorous-containing fat molecule that's found quite normally throughout the body, particularly in the membranes of blood cells and in the walls of blood vessels.

Phospholipids contribute to blood clot formation and play an important role in maintaining proper blood consistency. Due to the phospholipids being attacked by the antibodies, the blood becomes excessively 'sticky', resulting in a higher risk of developing blood clots.

Why is the immune system overactive?

We are not sure why the immune system becomes over-active and is unable to distinguish between foreign invaders and its own body constituents, but there is increasing clinical evidence that there is probably a genetic tendency which is triggered in people with the condition.

Some APS patients will have family members who also have the syndrome or have histories of other autoimmune diseases in their families such as lupus or thyroid disease. As well as lupus, other well known autoimmune diseases include multiple sclerosis, rheumatoid arthritis, and type 1 diabetes.

What triggers the disease?

Again, at this stage, it is unclear but a person's genetic make-up and exposure to certain trigger factors may provide the right environment in which APS can develop. One theory is that viral infections, such as glandular fever and shingles, stress, certain drugs, and other well known precipitants of thrombosis such as immobility, dehydration, surgery, the oral contraceptive pill and pregnancy can act as the trigger for the syndrome.

Related conditions

Some people with APS will also have other autoimmune conditions. View the main APS autoimmune 'cousins'

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