Abstract: The invention relates to a system for treating a biological fluid such as blood or a blood component by selective removal of a target substance, comprising a treatment bag (2) provided with at least one inlet orifice (3) and at least one outlet orifice (4), said treatment bag containing a set of particles (5) having an affinity for the target substance, said set comprising small particles having a size of less than 15 ?m, said system further comprising a barrier means (6) composed of at least one porous membrane made of a hydrophilic material, the pores of said membrane being calibrated to a size suitable for preventing the passage of said small particles, namely less than 3 ?m.