CORRESPONDENCE 2277 Antigenic Specificity of the To the Editor:

We read with great interest the thorough review by Roubey ’ on the antiphospholipid syndrome. However, we would like to comment on one point. To explain why autoantibodies to pure &glycoprotein I (O,-GPI) may be detected by enzyme-linked immunosorbent assay (ELISA) only when &GP1 is coated on irradiated plates, Roubey supports the first possibility, ie, that the antibodies require a rela-tively high density of immobilized antigen that cannot be achieved on untreated polystyrene. We present data that support the second possibility, ie, that the antibodies are specific for a conformational epitope of &GP1 formed when the protein is bound to an anionic surface, such as irradiated polystyrene or cardiolipin. Two monoclonal antibodies were used. c003 is a mouse mono-clonal antihuman &GP1 antibody developed by immunizing mouse with human &GP1 emulsified with Freund‘s complete adjuvant. WB-CAL-l is a monoclonal “anticardiolipin ” antibody derive