Filter system for filtering a fluid, comprising - a filter housing made of a housing upper part and a housing lower part which extend along a housing axis, - an inlet port disposed at the filter housing for supplying a fluid to be filtered, - an outlet port disposed at the housing lower part, in particular concentrically, with respect to the housing axis for discharging the filtered fluid, - a central tube disposed concentrically with respect to the housing axis, which central tube is fluidically connected to the outlet nozzle in at least one operating state, and which features a radially open area on at least one part of its axial length, - a secondary element, which is tubularly designed with a tubular wall and disposed inside the central tube, wherein the wall of the secondary element features a filter medium at least in some areas, - a filter element which separates a raw fluid side from a clean fluid side in a fluid-tight manner, wherein the filter element comprises a filter bellows disposed around its longitudinal axis with a first end disc at a first front face and a second end disc at a second front face, wherein the end discs (14, 16) feature central openings (15, 17) for receiving the central tube in the interior of the filter element, characterized in that the secondary element is closed at an end facing away from the outlet port, in that the secondary element features an axial length which is at least as large as the part of the axial length of the radially open area of the central tube and in that the secondary element is displaceably disposed inside the central tube along the housing axis between at least two operating positions (P1, P2) and opens, in the first operating position, a fluid path through the open area of the central tube and closes, in the second operating position, the fluid path through the open area while the secondary element covers the open area and the fluid flows through the wall of the secondary element from radially outside. - a filter housing made of a housing upper part and a housing lower part which extend along a housing axis, - an inlet port disposed at the filter housing for supplying a fluid to be filtered, - an outlet port disposed at the housing lower part, in particular concentrically, with respect to the housing axis for discharging the filtered fluid, - a central tube disposed concentrically with respect to the housing axis, which central tube is fluidically connected to the outlet nozzle in at least one operating state, and which features a radially open area on at least one part of its axial length, - a secondary element, which is tubularly designed with a tubular wall and disposed inside the central tube, wherein the wall of the secondary element features a filter medium at least in some areas, - a filter element which separates a raw fluid side from a clean fluid side in a fluid-tight manner, wherein the filter element comprises a filter bellows disposed around its longitudinal axis with a first end disc at a first front face and a second end disc at a second front face, wherein the end discs (14, 16) feature central openings (15, 17) for receiving the central tube in the interior of the filter element,