The airbag comprises an outer skin (20) having a first opening (26), a flap (32) being at least indirectly attached to the outer skin (20), and a tether (37). The first end of the tether is attached to a flap (32) and the second end (37b) of the tether is detachably attached to a holding element (16). When the airbag is deployed and the tether (37) is under tension, the flap (32) covers the first opening (26) at least partially such that the ventilation device is in a closed or throttled first state. When the airbag is deployed and the tether (37) is not under tension because the second end (37b) of the tether is detached from the holding element, the flap (32) covers the first opening (26) less than in in the first state, such that the ventilation device is in second state. The flap (32) is located between the outer skin (20) and an additional layer (34) having a second opening (36) at least partially overlapping the first opening (26).