HIT antibody binding in ELISA to the chimeric PF4-NAP-2 proteins complexed to heparin. (A) Binding of 50 HIT sera samples to WT PF4, WT NAP-2, and the PF4/NAP-2 chimeric proteins complexed to heparin. (B) Similar to (A), but only the 18 HIT sera with greatest reactivity to NNPP in (A) were studied. Each point represents binding activity of an individual serum sample. IgG binding is measured by OD₄₀₅. The mean OD₄₀₅ readings are indicated by a horizontal line for each protein. The mean OD₄₀₅ for the 20 normal sera tested with PF4/heparin complexes is shown as a dashed line.