VLA-4 expression on circulating leukocytes complexed to platelets after allergen exposure is increased. 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. Leukocytes were identified by forward and side scatter characteristics. Selected groups of thrombocytopenic mice were intravenously administered FUSPs or FSPs. Platelet–leukocyte complexes were identified with positive fluorescence gating for anti–CD41-FITC, and VLA-4 expression was then identified using anti–CD49d-streptavidin-PE conjugate. (A) VLA-4 expression on circulating leukocytes. (B) VLA-4 expression on platelet–leukocyte complexes. (C) VLA-4 expression on non–platelet-bound leukocytes. n = 5-9 animals per group. Data are expressed as mean ± SEM. ^*P < .05.