An isolation system is disclosed which includes a production screen and an internal isolation pipe sealed with the production screen at proximal and distal ends, and an internal sleeve slideably coupled with the isolation pipe. The isolation pipe defines at least one port and the sleeve defines at least one aperture, and the sleeve is moveable between an open position in which the port and aperture are in communication to permit fluid flow therethrough, and a closed position in which the port and aperture are not in communication and fluid flow is prevented. The sleeve is manipulated by a service string and multi-action shifting tool between the opened and closed positions. Also disclosed is a gravel packer and method of operation incorporating the isolation system, as well as a service tool and service string assembly useful therewith.