Abstract: A fluid processing assembly is used in association with a cassette that conveys fluid and a holder to hold the cassette. A door is carried by the holder for movement between an opened position to expose the holder and a closed position enclosing the cassette in the holder. The door carries an expandable gasket. The expandable gasket normally rests, when the door is in the closed position, in a relaxed state out of sealing contact with the held cassette. An actuator applies pressure to the expandable gasket to achieve, when the door is in the closed position, an expanded state in sealing contact against the held cassette. The assembly also includes a latch assembly on the door and holder, which makes engagement when the door is in the closed position. The engagement is unlocked when the gasket rests in the relaxed state. The engagement becomes locked when the gasket achieves the expanded state.