The invention provides a long range ordered, layered double hydroxide compound for use in modulating an immune response to an antigen, and an immune modulator composition comprising the layered double hydroxide and an interlayer anion A. The compound is [MII(1-x)MIIIx(OH)2]x+[An-x/n], in which MII is a divalent metal cation or a mixture of two or more divalent metal cations, MIII is a trivalent metal cation or a mixture of two or more trivalent meta cations, or a mixture of one or more trivalent metal cation with one or more quadravalent metal cation, x is from 2:1 to 1:4, and A is an interlayer anion having a charge n, wherein optionally some or all of the interlayer anion A may be replaced by one or biologically active agent and/or one or more antigen, and wherein A is selected from conjugate bases of acids having a pKa of −4 or higher.