Until recently the cause of TTP remained elusive. However, recent research points to the involvement of a protein in the plasma called von Willebrand factor (vWF).

vWF is a normal component of plasma, the straw-coloured fluid in which blood cells are suspended. It is required for effective blood clotting, and deficiency results in von Willebrand's disease, an inherited condition characterised by excessive bleeding.

Von Willebrand factor (vWF) is an extremely large molecule composed of identical subunits (multimers). Each multimer is able to bind to platelets or damaged endothelium (lining of blood vessels) at the site of an injury. The greater the number of multimers, the more effective is the binding. Ultra-large molecules of the factor (ULvWF) are therefore especially sticky but are not usually found circulating in the blood. Instead, they are normally broken down to slightly smaller sizes, so vWF retains its adhesive properties without binding inappropriately to platelets and causing undesired clots.

In TTP, vWF is synthesised normally, initially as ULvWF but its subsequent break down (cleavage) is defective. This is due to a lack of enzyme activity, called vWF cleaving protease, that breaks down von Willebrand factor in the blood. This deficiency may be inherited (genetic), in which case it will be revealed in childhood or it can be acquired later in life. Most adult-onset TTP appears to be secondary to the development of an antibody that inhibits this enzyme activity, whereas the childhood form is due to a simple reduction in enzyme activity. Both mechanisms result in the presence of ultra-large von Willebrand factor within the circulation. Circulating ULvWF leads to the inappropriate formation of platelet clumps (thrombi) particularly within blood vessels supplying the brain and kidneys. These give rise to the typical symptoms of TTP.