Revisiting beta 2 glycoprotein I, the major autoantigen in the antiphospholipid syndrome

Beta 2 glycoprotein I (β2GPI) is a single chain 50 kDa highly glycosylated glycoprotein at an approximate concentration of 4 μM in cells. The abundance of this protein in plasma and its high state of preservation indicate the important role of this protein in mammalians. In addition, β2GPI has a particular structure in the fifth domain, and is categorized as the major antigen recognized by autoantibodies present in antiphospholipid syndrome. Beta 2 glycoprotein I has been usually studied in the context of antiphospholipid antibody production. Complexes of β2GPI/anti-β2GPI antibodies have been examined in different coagulation and cell activation pathways. However, the function of β2GPI, independent from the antibodies, has not been clearly determined. In this paper different features of β2GPI including its structure, plasma concentration and its proposed in vitro and in vivo functions in clot formation and fibrinolysis along with anti-β2GPI antibodies (Abs) are discussed. Their inhibitory or promotive effects are delineated in each facet.