The degree of neutrophil activation induced by anti-PF4/heparin ICs is determined, in part, by neutrophil count. (A) The KKO-PF4/heparin IC activation phenotype is determined in the cellular fraction. Whole blood from an established high-reactivity donor and an established ABO-compatible low-reactivity donor was separated into their cellular and plasma fractions by centrifugation. After mixing the cellular and plasma fractions from high- and low-reactivity donors, reconstituted blood was incubated with buffer, PF4 (25 µg/mL), heparin (1 U/mL) alone or in the presence of KKO or isotype control (Iso, 25 µg/mL). MMP-9 release from reconstituted blood was compared with unmanipulated whole blood from the 2 donors. Results are expressed as mean ± standard deviation values for triplicate wells. (B) MMP-9 release in healthy donors is correlated with WBC count. WBCs from healthy donors (n = 71) is plotted as a function of KKO-PF4/heparin IC-induced MMP-9 release (Spearman r = 0.39; P = .001). (C) MMP-9 release in healthy donors is correlated with ANC. ANC in healthy donors (n = 71) is plotted as a function of KKO-PF4/heparin IC-induced MMP-9 release (Spearman r = 0.49; P < .0001). (D) Within healthy donor quartiles, MMP-9 release does not correlate with ANC. Healthy donors were divided into quartiles based on their KKO-PF4/heparin IC neutrophil response (as shown in Figure 5A). Within each quartile, ANC was plotted as a function of MMP-9. P = ns for all groups.