The table shows the differences in onset of the immune response (in days) between the Ig classes IgG, IgM, and IgA in the 58 patients who reacted with at least 2 Ig classes to PF4/heparin complexes. Most IgG and IgM immune responses first became detectable on the same day or within only 1 day. This indicates that the pattern of the immune response in HIT does not follow the pattern of either a classic primary immune response (ie, initial formation of IgM antibodies, followed several days later by IgG antibodies) or a secondary immune response (ie, primarily IgG antibodies with an associated weak IgM response).