Systems and methods separate blood cells from whole blood and pump theseparated blood cells through an in-line leukofilter to a blood cell storagecontainer. The leukofilter comprises a filtration medium enclosed within aflexible housing. The systems and methods include a fixture to restrainexpansion of the flexible filter housing during operation of the pump. Thefixture includes a bracket to enable its releasable attachment to the bloodprocessing device employed to carry out the separation process.