Antiphospholipid-Antikörper (Prothrombin, β2-Glykoprotein I): the pathway in the body

Antiphospholipid-Antikörper (Prothrombin, β2-Glykoprotein I) is part of the pathway “Antiphospholipid antibodies”. This page shows the whole pathway; the station of Antiphospholipid-Antikörper (Prothrombin, β2-Glykoprotein I) is highlighted.

Where this laboratory value sits: Antibodies on cells — on vessel and blood cells. The antibody complexes bind to cells that carry phosphatidylserine or suitable receptors: the vessel wall, monocytes and platelets. Source 1, 2

In brief

Antiphospholipid antibodies are autoantibodies against blood proteins that bind to membrane lipids, mainly β2-glycoprotein I and prothrombin. Once bound, they cross-link these proteins on cell surfaces and trigger signals there that set coagulation in motion.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Phosphatidylserine → β2GPI on the membrane · β2-glycoprotein I β2-glycoprotein I (β2GPI) mostly circulates in the blood in a closed, ring-like shape. When it attaches by one end to negatively charged surfaces, it opens up. Source 3, 2↕ both, depending on amount β2GPI participates in the fine-tuning of coagulation and in clearing dead cells; its exact function is not yet fully understood. observed in studies Source 3
   ⚖ When the balance tips

   too much — If a lot of β2GPI lies opened on membranes, the site to which antibodies against β2GPI bind becomes exposed.

   too little — If little lies on membranes, β2GPI stays closed and the antibodies' binding site stays hidden.

   observed in studies · Source 2, 3

2. β2GPI on the membrane → Complex with anti-β2GPI · anti-β2GPI (IgG) Antibodies against β2GPI mostly bind to its first domain and link two molecules together. The complex adheres more firmly to the membrane. Source 2, 1↓ depletes The cross-linked complex occupies the phosphatidylserine surface; in laboratory experiments it displaces annexin A5, which otherwise shields such surfaces from clotting factors. observed in studies Source 2, 6
   ⚖ When the balance tips

   too much — If many complexes form, they bind more often to receptors on endothelial cells, monocytes and platelets.

   too little — If few form, β2GPI stays mostly single and the receptors are hardly engaged.

   observed in studies · Source 2, 1

3. Prothrombin → Prothrombin on PS · calcium With its Gla residues and calcium ions, prothrombin binds to phosphatidylserine (PS). There it is converted to thrombin. Source 5, 4↑ supplies On the membrane, prothrombin and the enzymes that cleave it lie close together; this speeds up thrombin formation many times over. established physiology Source 5
   ⚖ When the balance tips

   too much — If a lot of prothrombin is bound to PS, a binding site forms there that exists only in the complex and to which anti-PS/PT antibodies bind.

   too little — If calcium is absent or the Gla residues are incomplete, prothrombin hardly binds to the membrane.

   observed in studies · Source 4, 5

4. Prothrombin on PS → Complex with anti-PS/PT · anti-PS/PT (IgG) Antibodies against prothrombin recognise it alone or in complex with phosphatidylserine. Once bound, they hold prothrombin on the membrane. Source 4, 1↕ both, depending on amount Bound antibodies can concentrate prothrombin on the surface; in laboratory clotting tests, by contrast, they often prolong clotting time. observed in studies Source 4, 1
   ⚖ When the balance tips

   too much — If many complexes form, more prothrombin is held on membranes, and the antibody is found more often together with other antiphospholipid antibodies.

   too little — If few form, prothrombin stays freely mobile and follows the normal course of coagulation.

   observed in studies · Source 4

5. Complex with anti-β2GPI → Antibodies on cells The antibody complexes bind to cells that carry phosphatidylserine or suitable receptors: the vessel wall, monocytes and platelets. Source 1, 2↓ depletes On the cells the complexes trigger signals and activate the complement system; both shift the balance of coagulation towards fibrin formation. observed in studies Source 1
   ⚖ When the balance tips

   too much — If many complexes bind, more cells are activated, and complement fragments attract further immune cells.

   too little — If few bind, the cells largely stay at rest.

   observed in studies · Source 1

   Field of research — Antiphospholipid antibodies are studied in disorders of blood coagulation and in the course of pregnancy. Source 1, 4

6. Complex with anti-PS/PT → Antibodies on cells The antibody complexes bind to cells that carry phosphatidylserine or suitable receptors: the vessel wall, monocytes and platelets. Source 1, 2↓ depletes On the cells the complexes trigger signals and activate the complement system; both shift the balance of coagulation towards fibrin formation. observed in studies Source 1
   ⚖ When the balance tips

   too much — If many complexes bind, more cells are activated, and complement fragments attract further immune cells.

   too little — If few bind, the cells largely stay at rest.

   observed in studies · Source 1

   Field of research — Antiphospholipid antibodies are studied in disorders of blood coagulation and in the course of pregnancy. Source 1, 4

7. Cell receptors → Tissue factor p38, NF-κB Tissue factor is the protein that starts coagulation. Normally it is hardly present on cells in contact with the blood. Source 1↑ supplies Tissue factor binds factor VIIa; this complex activates factor X, which together with factor Va cleaves prothrombin to thrombin. established physiology Source 1, 5
   ⚖ When the balance tips

   too much — If many cells carry tissue factor, coagulation starts at more sites at the same time.

   too little — If few cells carry tissue factor, coagulation starts only where a vessel wall is injured.

   observed in studies · Source 1

8. Tissue factor → Thrombin factor VIIa, Xa Factor Xa cleaves prothrombin to thrombin on the membrane. Thrombin is the key enzyme of coagulation. Source 5↕ both, depending on amount Thrombin converts fibrinogen to fibrin and activates platelets; at the intact vessel wall it also switches on the brake of coagulation via protein C. established physiology Source 5
   ⚖ When the balance tips

   too much — If a lot of thrombin forms, more fibrin is produced and thrombin amplifies its own formation via further clotting factors.

   too little — If little thrombin forms, fibrin forms more slowly and platelets are activated more weakly.

   established physiology · Source 5

9. Thrombin → Fibrin · fibrinogen Thrombin cleaves fibrinogen; the resulting fibrin strands assemble into a mesh that holds platelets and blood cells in place. Source 5↑ supplies The fibrin mesh seals injured vessels; later the enzyme plasmin dissolves it again. established physiology Source 5
   ⚖ When the balance tips

   too much — If a lot of fibrin forms, the mesh grows beyond the site of injury until plasmin and coagulation inhibitors counteract it.

   too little — If little fibrin forms, the mesh stays loose and a vessel is sealed more slowly.

   established physiology · Source 5

Further stations

• Phosphatidylserine — outside on activated cells Phosphatidylserine is a negatively charged membrane component that normally faces inwards. Activated platelets and dying cells turn it outwards. Source 5, 3↑ supplies Phosphatidylserine turned outwards is the docking surface on which clotting factors assemble and prothrombin is rapidly converted to thrombin. established physiology Source 5
  ⚖ When the balance tips

  too much — If a lot of phosphatidylserine faces outwards, more clotting factors and β2-glycoprotein I bind to these surfaces.

  too little — If little faces outwards, the clotting factors find hardly any surface and thrombin forms slowly.

  established physiology · Source 5, 3

• Prothrombin — precursor of thrombin Prothrombin is made in the liver and circulates in the blood as a precursor. Vitamin K provides it with Gla residues that bind calcium. Source 5↑ supplies Prothrombin is the reserve from which thrombin, the key enzyme of coagulation, forms when needed. established physiology Source 5
  ⚖ When the balance tips

  too much — If a lot of prothrombin is present, more thrombin can form quickly on an activated surface.

  too little — If little is present, less thrombin forms on the same surface.

  established physiology · Source 5

• Cell receptors — ApoER2, TLR4, annexin A2 The cross-linked β2GPI complex binds to receptors such as ApoER2, TLR4 and annexin A2. Inside the cell, signalling pathways such as p38 and NF-κB then become active. Source 1, 2↑ supplies The signals switch on genes for tissue factor and adhesion molecules in endothelial cells and monocytes and activate platelets. observed in studies Source 1
  ⚖ When the balance tips

  too much — If many receptors are occupied, endothelial cells and monocytes make more tissue factor and adhesion molecules.

  too little — If few are occupied, tissue factor production stays at its resting level.

  observed in studies · Source 1

Cofactors in this pathway

• Vitamin K — Forms the Gla residues on prothrombin with which it binds to phosphatidylserine via calcium Source 5
• Calcium — Links the Gla residues of prothrombin to the phosphatidylserine surface Source 5
• Phosphatidylserine — Negatively charged membrane surface to which β2GPI and prothrombin bind Source 3, 5
• Fibrinogen — Is cleaved by thrombin to fibrin, which forms the mesh of the clot Source 5
• Annexin A5 — Shields phosphatidylserine surfaces from clotting factors Source 6

Sources

1. Giannakopoulos B, Krilis SA. The pathogenesis of the antiphospholipid syndrome. N Engl J Med 2013 · PubMed 23484830
2. de Groot PG, Urbanus RT. The significance of autoantibodies against β2-glycoprotein I. Blood 2012 · PubMed 22553312
3. de Groot PG, Meijers JC. β(2) -Glycoprotein I: evolution, structure and function. J Thromb Haemost 2011 · PubMed 21535391
4. Sciascia S, Sanna G, Murru V et al. Anti-prothrombin (aPT) and anti-phosphatidylserine/prothrombin (aPS/PT) antibodies and the risk of thrombosis in the antiphospholipid syndrome. A systematic review. Thromb Haemost 2014 · PubMed 24172938
5. Krishnaswamy S. The transition of prothrombin to thrombin. J Thromb Haemost 2013 · PubMed 23809130
6. Rand JH, Wu XX, Quinn AS, Taatjes DJ. Resistance to annexin A5 anticoagulant activity: a thrombogenic mechanism for the antiphospholipid syndrome. Lupus 2008 · PubMed 18827057

Whole pathway: Antiphospholipid antibodies

Related pathways

• Clotting and D-dimers — prothrombin

As of 2026-10-05. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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