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While the anti-phospholipid antibodies are pathognomonic of the Anti-Phospholipid Syndrome, some may also be present in other rheumatic diseases without thrombophilic complications.
The putative epitope for APS is now recognised as Domain 1 of the 5-domains beta2-glycoprotein I (β2GPI).
This study found anti-D1, anti-D4/5 and anti-phosphatidylglycerol antibodies to be helpful in differentiating APS from the other rheumatic diseases.
If may be speculated that binding of D4/5 to cell membrane negatively charged phosphatidylglycerol results in a conformational change in β2GPI, exposing D1 and triggering autoimmune thrombosis.