A blow-by gas filtration assembly fluidically connects to an internal combustion engine crankcase venting circuit. The filtration assembly has an axis and a main body including a filtration chamber with a flow aperture delimited by an aperture edge concentric to the axis. The filtering assembly includes a filter group; including a filter cavity crossed by blow-by gases; a command group connected to the filter group; an annular collar connected to the filter group; an annular alignment and fixing group between the aperture edge and the annular collar, for coupling the filter group to the main body. A dynamic element includes a first radial face engaged at the aperture edge and a second radial face radially facing the annular collar; an annular element between the dynamic element and the annular collar. The annular element elastically yields to a flattened configuration compressed between the second radial face and the annular collar.