Platelet Factor 4 (PF4, CXCL4) is an ELR^- tetrameric, cationic chemokine that constitutes ~25% of the protein mass of platelet α-granules. It is also found bound to the luminal vascular endothelial surface. Treatment with heparin results in PF4-heparin complex formation and the release of PF4 into blood. In 1-5% of patients, the formation of PF4-heparin complexes in blood triggers the generation of anti-PF4 antibodies, and a bleeding disorder called heparin-induced thrombocytopenia (HIT). Paradoxically, about 10% of patients with HIT also experience life-threatening thrombosis. Since platelet activation is often associated with changes in the function of phagocytotic blood cells, we examined the effect of PF4 on human neutrophils both in the presence and absence of anti-PF4 antibodies. Studies were performed both with PF4 isolated from human platelets (hPF4) and recombinant (rPF4) sources. We observed that while PF4 or anti-PF4 antibody alone did not alter neutrophil function, costimulation with anti-PF4 antibody and human PF4 resulted in ~3-fold increase in cell surface leukocyte Mac-1 expression, and degranulation of leukocyte secondary and tertiary granules. The level of Mac-1 upregulation peaked at an intermediate PF4 dose suggesting that functional response varies with antigen-antibody stoichiometry. Confocal microscopy studies show the formation of PF4 anti-PF4 immune complexes in these runs and their co-localization with Fc_γRII receptor, CD32. Functional studies further demonstrate a role of leukocyte surface chondroitin sulfates in stabilizing the binding of immune complex to CD32, and a role of signaling via CD32 in inducing cell activation. Neutrophil activation enhanced cell adhesion via adhesion molecules belonging to the selectin and integrin families. Studies with recombinant protein suggest a key role for PF4 C-terminal amino-acids in inducing leukocyte activation. Similar experimental observations were made with a range of monoclonal antibodies against PF4 and purified IgG from HIT patients. Together, the findings suggest that similar activation mechanisms (i.e. engagement of PF4 anti-PF4 immune complexes with the CD32 receptor) can trigger both platelet and neutrophil activation during HIT. Since activated neutrophils adherent on the vessel wall may damage the endothelium and initiate the process of thrombosis and vasculitis, our observation may be relevant to the pathogenesis of thrombosis in patients mounting an immune response to PF4-heparin.