The invention relates to compounds, in particular polypeptides that specifically bind to the non-classical MHC protein CD1d and modulate CD1d-mediated biological functions. The invention in particular relates to such compounds and polypeptides comprising or consisting of at least one single domain antibody, and wherein at least one single domain antibody specifically binds to CD1d. Also provided is for methods and use employing such compounds, polypeptides and/or single-domain antibodies.