Heparin-induced thrombocytopenia (HIT) is caused by platelet-activating antibodies that recognize PF4/heparin complexes. Uncertainties remain regarding HIT immunobiology, including the temporal relation of antibody formation to onset of thrombocytopenia, and whether immunoglobulin class switching occurs. Using serial plasma samples from 2 heparin thromboprophylaxis trials, we determined the time of onset, antibody levels, and immunoglobulin class distributions (IgG, IgA, IgM) for 12 patients with HIT and 36 patients who formed anti-PF4/heparin antibodies, but did not develop HIT (“seropositive non-HIT controls”). In patients with HIT, anti-PF4/heparin antibodies became detectable 4 days (median) after starting heparin; antibody detection preceded the platelet count decline by 2 days (median). Patients with HIT produced higher levels of IgG antibodies, but similar IgA and IgM levels, compared with seropositive non-HIT controls. Among all 48 seroconverting patients, the first day of a positive antibody test (median, day 6) did not differ among the immunoglobulin classes. Thus, the HIT immune response does not exhibit the classic paradigm of IgM class precedence/immunoglobulin class switching; rather, relatively rapid formation of IgG antibodies is observed, sometimes with concomitant IgA and IgM formation. Compared with seropositive non-HIT controls, HIT patients develop significantly higher anti-PF4/heparin IgG levels that are detectable before the onset of thrombocytopenia.