A device for reducing the number of leucocytes in blood comprising a plurality of hollow fibers based on organic polymers, whereby the hollow fibers have a lumen and a wall surrounding the lumen, said wall having an internal surface facing the lumen and an external surface, whereby the hollow fibers are arranged in a cylindrical housing with an inlet arrangement and an outlet arrangement and whereby an outer space is formed between the hollow fibers and the housing, which space is accessible for a fluid via the inlet arrangement and the outlet arrangement, characterized in that only the external surfaces of the hollow fibers are accessible for a fluid, that the lumina of the hollow fibers are not accessible for a fluid, that the arrangement of the hollow fibers shows a high degree of order and that the hollow fibers based on organic polymers cause a generation of the complement activation product C5a in a concentration of at least 10 μg per m2 of fiber surface.