HIT antibody binding to the K to A mutant PF4 proteins. Binding of 50 antibodies from HIT patients to WT PF4, WT NAP-2, and K to A mutant proteins complexed with heparin was tested in ELISA, as described in Materials and Methods. Each point represents binding activity of an individual plasma sample. Results are shown as light absorbency at 405 nmol/L. The mean OD₄₀₅ readings are indicated by a horizontal line for each protein tested. The mean OD₄₀₅ for the 20 normal sera tested with PF4/heparin complexes is shown as a dashed line.