Blood collection systems and filters using a porous membrane element

The invention generally relates to blood collection and processing systems and methods. A filter for removing leukocytes from blood comprises a filter media enclosed in a housing. The filter media includes a main filter region comprising a porous membrane structure extending between first and second skin surfaces. The porous membrane structure is formed by intersecting cells having a range of diameters. The cells adjacent to the first skin surface have diameters generally smaller than the diameters of the cells adjacent to the second skin surface. The first skin surface includes an open area defined by pores, which are formed by the intersection of cells with the first skin surface. The majority of the open area is defined by pores having a diameter of between about 12 m and 28 m.