wherein A is CH or N, p is in a range from 1 to 50, R″ is a bond or -alkylene-O—, R′ is alkylene that is optionally interrupted or terminated with one or more amide or ether groups, and E is an amine- or thiol-reactive group. Pharmaceutical compositions containing the compound or the conjugate, methods of making a conjugate, and methods of use of the compounds or conjugates as immunomodulators for inducing cytokine biosynthesis in an animal and for vaccinating an animal are also disclosed. An antigen modified by the linker is also disclosed.