Figure 1: Long-term filtration during extracorporeal circulation (ECC) not only removes but also activates leukocytes. A closed ECC loop was established with a regular arterial filter (Group C) or a leukocyte filter (Group S or L, in each group). The ECC was primed with 300 mL of blood. Blood in Group S was filtrated for 5 min, then the leukocyte filter was bypassed, while blood in Group L was filtrated throughout the 60 min experiment. Leukocytes (a), neutrophils (b), and lymphocytes (c) were reduced after 5 min of filtration in groups L and S. However, mean fluorescent intensities (MFI) of CD11/CD18 of leukocytes (a′), neutrophils (b′), and lymphocytes (c′) were significantly higher in Group L than in groups C and S. * and ** versus control; and # and ## versus Group L.