Anti-PF4 antibody characterization in spHIT, HIT, and patients with ELISA-positive but non-activating anti-PF4 antibodies (FP-HIT). (A-B) Eluates from PF4-heparin beads and control heparin beads were evaluated for platelet activation in the PEA and PF4-polyanion ELISA. Means and SD (n = 3) are shown. (C-F) Shown are LC-ESI-QTOF MS +11 light chain distributions from anti-PF4 antibodies isolated from patients with (C) spHIT, (D) HIT, and (E-G) FP-HIT. In the spectra, green represents the distribution of all λ-containing Ig’s, red represents the distribution of all κ-containing Ig’s, and blue represents the light chain distribution of κ and λ light chains associated with an IgG heavy chain. The numbers listed above peaks depict the identified light chain’s m/z ratio. The x-axis shows m/z ratios, and the y-axis depicts the relative abundance of the monoclonal or oligoclonal antibody identified.