Avidity of anti-beta-2-glycoprotein I antibodies

The terms affinity and avidity are often used indiscriminately, despite clearly differing. Since affinity refers to monovalent binding of antibodies to a monovalent epitope, the majority of data on the binding of anti-β2-glycoprotein I antibodies (anti-β2-GPI) characterized their avidity rather than affinity. Anti-β2-GPI were generally believed to be of low avidity, but heterogeneous avidity of patientsʹ IgG anti-β2-GPI has been demonstrated. High avidity anti-β2-GPI monoclonals were reported to possess higher pathogenicity than low avidity anti-β2-GPI. Polyclonal high avidity anti-β2-GPI were found to be more common in patients with antiphospholipid syndrome (APS) and associated with thrombosis. Some conformational changes of β2-GPI are required for the binding of polyclonal anti-β2-GPI to the antigen: neither high density of the antigen nor high avidity of the anti-β2-GPI alone is sufficient for the recognition. Avidity of anti-β2-GPI should be considered in any attempt of inter-laboratory standardisation and/or evaluation of anti-β2-GPI enzyme-linked immunosorbent assay (ELISA).