Inhibition of HIT antibody binding to WT-PF4/heparin complex by chimeric proteins and by anti–C-terminal PF4 antibodies. HIT antibody sample in a predetermined concentration giving 50% binding to WT PF4/heparin was preincubated with various WT or chimeric proteins at different concentrations. Residual antibody binding to WT PF4/heparin was then measured in ELISA and expressed as HIT antibody binding: % of control (100% binding without inhibitor). Note the similar maximal inhibition obtained in the presence of WT PF4/heparin and the chimera NNPP/heparin. (⧫) BSA; (▴) NAP-2; (▧) NNNP; (▾) PF4; (•) NNPP.