Allogeneic hematopoietic stem cell transplantation (HSCT) remains the only curative treatment in most haematological malignancies. However, its anti-tumor effect (GVT) is often associated with the development of a graft versus host disease (GVHD) and impaired immune anti-infectious responses. The predictive and protective features of iNKT cells on the development of acute GVHD (aGVHD) are not or poorly known in humans and their mechanisms of action, in particular their potential interactions with the other regulatory effectors and immune actors of the allogeneic response, remain to be explored.