Disclosed is an apparatus for isolating and separating leukocytes from red blood cells in a whole blood sample, the apparatus comprising: a filtration device comprising an upper chamber, which is a cylindrical tube, a lower chamber, which is a cylindrical tube, and one or more stacked filter membranes between the upper and lower chambers, wherein the one or more stacked filter membranes are capable of retaining the leukocytes, wherein the inner diameters of the upper and lower chambers are similar as to create a cylindrical tube which allows liquids to pass there through, wherein the one or more stacked filter membranes is placed between screw threads of the upper chamber and lower chamber, which chambers are securely attached together； and a collection tube for collecting red blood cells from the whole blood sample, wherein the filtration device is placed on top of the collection tube, and wherein the red blood cells are separated from the leukocytes and collected in the collection tube following centrifugation. Also disclosed is a method for preparing a lysate of leukocytes from a whole blood sample by use of the apparatus.
