Amorphous magnesium-substituted calcium, phosphate compositions and their medical uses are described, in particular for use in delivering cargo materials, such as cargo molecules or cargo nanoparticles contained in pores of the amorphous magnesium-substituted calcium phosphate to cells of the immune system, for example as therapeutic approaches for the treatment of inflammatory bowel diseases, and in particular Crohn' s disease, autoimmune diseases, allergy and for therapeutic vaccination.