PF4 and anti-PF4/heparin antibodies enhance bacterial phagocytosis. FITC-labeled E coli were preincubated with hu-PF4 (black and gray bars) or with buffer (open bars) and additionally with heat-inactivated human serum, which did (black bar) or did not (gray bar) contain anti-PF4/heparin IgG (preadsorbed with E coli alone), or with buffer alone (open bar). After preincubation, the bacteria were subjected to a whole blood phagocytosis assay using flow cytometry. The figure shows the MFI of FITC-positive PMNs, which is a marker for phagocytosed E coli. Data are mean ± SD of 4 different sera representative of 2 independent experiments. **P < .01. n.s. indicates not significant.