The protumorigenic role of tumor-derived exosomes. Tumor-derived exosomes help create an immunosuppressive tumor microenvironment by inducing apoptosis and impairing the function of effector T cells and NK cells, skewing DC differentiation into MDSCs as well as promoting Treg cell activity. They also contribute to the establishment of a pre-metastatic niche by enhancing angiogenesis, remodeling stromal cells, and promoting extracellular matrix degradation. Tumor-derived exosomes also function as delivery vehicles to transfer microRNA and mRNA to neighboring cells. Moreover, tumor-derived exosomes can help tumor cells develop drug resistance by exporting tumoricidal drugs or neutralizing antibody-based drugs.