| http://www.w3.org/ns/prov#value | - A unit dose of a composition comprising a pharmaceutically acceptable diluent and an immune system-activating amount of a compound of the formula ##STR26## in which X is hydrogen or the radical CH2 OR5,R2, R3, R4 and R5 each independently is hydrogen or the radical ##STR27## Y is oxygen, sulphur, NH or CH2, and R1 and R6 each independently is a hydrocarbon radical having up to 50 carbon atoms, or
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