Day of first onset of immune response against PF4/heparin separately for IgG, IgM, and IgA antibodies. The figure shows the days during application of heparin during which the immune response against PF4/heparin complexes first became detectable, shown separately for IgG, IgM, and IgA antibodies. There was a narrow time period between day 4 and day 12 during which the immune response predominantly occurred. The timing of the onset of the immune response was the same for antibodies of the IgG, IgA, or IgM classes.