Anti-PF4/heparin IgG from sera of patients with HIT binds to PF4-coated bacteria but not to bacteria alone. From sera of 5 patients with HIT, known to contain anti-PF4/heparin antibodies, IgG antibodies were affinity purified using hu-PF4-precoated bacteria. (Insets) Depletion of the anti-PF4/heparin antibodies from the original serum (n = 5) by sequential incubation with hu-PF4–coated bacteria (mean ± SD). In the main panels, each symbol represents the reactivity of the affinity-purified antibodies of one serum; mean values are given as horizontal lines. Reactivities of antibodies affinity purified by hu-PF4-coated E coli JM 109 (A) or by hu-PF4-coated S aureus SA113spa (B). Reactivity of the purified antibodies with hu-PF4/heparin complexes (first column) was inhibited by excess heparin (100 IU/mL unfractionated heparin [UFH], column 2), which disrupts PF4/heparin complexes. Accordingly, antibodies did also not react with hu-PF4 alone (column 3). Non–PF4-coated bacteria served as control for unspecific binding of the antibodies to bacteria alone (column 4).