Newly published data indicate that patients with blinatumomab-sensitive B-precursor acute lymphoblastic leukaemia (B-ALL) have significantly fewer regulatory T cells (Treg) than those with blinatumomab-refractory B-ALL. This observation is attributed to IL-10 production by blinatumomab-activated Treg cells, resulting in suppression of T-cell proliferation, and highlights that Treg enumeration prior to treatment has the potential to identify 70% of nonresponders and thus might spare these patients from receiving an ineffective treatment.