Fig. 1

Preferential binding of anti-phospholipid antibodies (aPL) to domain 1 (D1) of patient’s beta2-glycoprotein I (β₂GPI) in the line immunoassay (LIA). In contrast to the planar solid phase used in enzyme immunoassays, the porous hydrophobic LIA membrane incorporates the hydrophobic phospholipid (PL)-tail during immobilization. This shields the by far larger tail of the amphiphatic PL molecule from the reaction environment and, thus, prevents unspecific interactions. Number, orientation, and accessibility of anionic phosphate groups of the differing hydrophilic PL heads may influence the binding of the patient’s β₂GPI (a) and consequently of the β₂GPI-dependent aPL (c). After binding of β₂GPI to the immobilized anionic PL by domain 5 (D5, containing the PL-binding site), D1 forms the accessible top of the induced fish-hook-like β₂GPI structure (b). Due to the high density of negatively charged PL heads on the membrane, the formation of a β₂GPI layer with a unique D1 epitope structure is assumed. The layer formation seems to hinder aPL binding to β₂GPI epitopes close to D5 [16]