In at least one embodiment, an air bag system comprises an air bag having an impact surface which in a non-inflated state is arranged in a housing located below the upper edge of a windowsill of a vehicle door. In an inflated state, at least part of the impact surface is positioned above the windowsill. There is a supporting surface for the air bag below the upper edge of the windowsill. The air bag defines a main chamber and a supporting chamber. The impact surface forms part of the main chamber. The supporting chamber, in an inflated state, is situated between the main chamber and the supporting surface and is supported at least in part by the supporting surface.