The present invention concerns a use of a MHC class II ligand, such as CD4 and LAG-3, for the manufacture of a medicament for preventing or treating pathological conditions involving an antigen specific immune response, as well as the use of LAG-3 in cancer-immunotherapy. The invention also concerns a pharmaceutical composition comprising an effective amount of an antigen able to induce an antigen-specific immune response together with an effective amount of a MHC class II ligand wherein said MHC class II ligand is present as an adjuvant-like agent.