The invention relates to a filtration unit intended to allow the selective eliminationof target substances from a fluid such as blood or a blood component, comprisingan outer envelope formed from two flexible sheets joined together over theirperiphery along a bond line so as to define an inner volume, the envelope containinga filtering medium which delimits, on both sides of said bond line, an inlet compartmentof the fluid to be filtered and an outlet compartment of the filtrate, each of thecompartments being provided with an element - respectively an inlet and outletelement - for bringing said compartment into communication with the outside,said elements being connected between the sheets and comprising a flow channelthat passes through the bond line, said unit being characterized in that at leastone channel is offset relative to the bond line towards the compartment with whichsaid channel is in communication.
