CD11b expression on circulating leukocytes is suppressed in mice depleted of platelets but is restored with the infusion of FSPs forming complexes with leukocytes. Sham- and OVA-sensitized mice were exposed to allergen, and circulating blood was obtained by cardiac puncture 24 hours after 1 day of allergen exposure. In some groups, mice were rendered thrombocytopenic with the administration of busulfan. Selected groups of thrombocytopenic mice were intravenously administered FUSPs or FSPs. Leukocytes were identified by forward- and side-scatter characteristics. Platelet–leukocyte complexes were identified with positive fluorescence gating for anti–CD41-FITC, and CD11b expression was then identified using anti–CD11b-PE fluorescence. (A) CD11b expression on circulating leukocytes. (B) CD11b expression on platelet–leukocyte complexes. (C) CD11b expression on non–platelet-bound leukocytes. n = 5-9 animals per group. Data are expressed as mean ± SEM. ^*P < .05; ^***P < .001.