Filtration unit for the selective elimination of a target substance

A filtration unit for the selective elimination of a target substance from a biological fluid comprising an external envelope, wherein the envelope encloses a filtering medium comprising: an upstream structure for eliminating at least the target substance comprising a stack of layers that are arranged to allow the flow of fluid through the layers in one direction comprising a non-woven material, and particles interposed between at least some of the layers, wherein the particles have an affinity for the target substance; and a downstream structure for retaining the particles comprising at least one layer of porous material.