Changes in the immune response against PF4/heparin over time. The mean ODs in the anti-PF4/heparin EIA for each day of heparin application are shown. Beginning at approximately day 5, the ODs first increased, peaking between day 7 and day 12, and then decreased again, despite continuing application of heparin. The mean peak OD values during the day 7 to day 12 interval were significantly higher than the ODs observed on the last day of heparin application (P < .001). This pattern is given for intermediate (0.5- < 1.0) and strong (> 1.0) maximal changes in OD. The x-axis represents the days of heparin application; y-axis, the mean ODs measured. All ODs were measured, whereas the respective patients received UFH or LMWH. For 7 of the 8 patients who exhibited a strong immune response (ΔOD ≥ 1.0), antibody levels declined after reaching a peak despite continued heparin administration.