In vitro activation of the contact activation system (Hageman factor system) in plasma by acidic phospholipids and the inhibitory effect of beta 2-glycoprotein I on this activation.

1. Negatively charged phospholipids promote initiation of the contact activation system in the blood coagulation. 2. Neutral phospholipids were unable to activate this system. 3. The activation is inhibited by beta 2-glycoprotein I at physiological concentrations. 4. The results raise the question whether people with low concentration of beta 2-glycoprotein I are more easily exposed to blood coagulation defects, such as disseminated intravascular coagulation, than those with normal concentration of beta 2-glycoprotein I.