A filter system for filtering fluid has a filter housing with inlet socket and outlet socket for fluid. A central tube with radially open region over a portion of its axial length is arranged in the filter housing in flow communication with the outlet socket in an operating state. Within the central tube, a secondary element is arranged slidably between first and second operating positions. A filter element, separating a raw fluid side from a clean fluid side, has a filter bellows with a first end plate at a first end face and a second end plate at a second end face. The end plates have central openings for receiving the central tube in the filter element interior. In the first operating position, the secondary element opens a fluid path through the radially open region of the central tube and closes the fluid path in the second operating position.