Significance Therapeutic antitumor antibodies are widely used clinically. CD47 is an antiphagocytic ligand expressed by tumors that binds the inhibitory receptor signal regulatory protein alpha (SIRPα) on phagocytic cells. Interruption of CD47–SIRPα interactions in immunodeficient mice bearing human tumors enhances therapeutic antitumor antibody responses by promoting phagocytosis of antibody-bound tumor cells. Here, we use a novel anti-CD47 single domain antibody, derived from an alpaca, in an immunocompetent mou…