Extracellular vesicles (EVs), including exosomes, are small membrane vesicles derived from multivesicular bodies or the plasma membrane. They contain proteins, lipids, and nucleic acids from the parent cell and play a crucial role in intercellular communication by transferring their contents between cells. EVs are involved in various physiological processes and have important roles in immune regulation. EV-based therapeutics are being developed for inflammatory and autoimmune diseases and cancer. This review focuses on the role of EVs in modulating immune responses and their therapeutic applications. EVs are generated by inward budding of the endosomal membrane, leading to the formation of intra-luminal vesicles (ILVs) within endosomes, which are then termed multivesicular bodies (MVBs). MVBs can follow either the secretory or lysosomal pathway. In the secretory pathway, MVBs fuse with the cell membrane, releasing ILVs as exosomes. In the lysosomal pathway, MVBs fuse with lysosomes, releasing ILVs for degradation. The generation of exosomes requires sorting of exosome-targeted proteins and lipids into the endosomal membrane, delivery of the exosome cargo into nascent ILVs, and excision of ILVs. Monoubiquitylation of cytosolic domains of transmembrane proteins functions as a sorting signal to direct them to ILVs. The ubiquitylated proteins are captured by the endosomal sorting complex for transport (ESCRT) machinery. MVBs can still be generated in the absence of key subunits of all ESCRTs, indicating the existence of alternative mechanisms of exosome biogenesis. EVs can directly or indirectly present antigens (Ag) to T cells. Direct Ag presentation involves EVs carrying MHC Class-I and MHC Class-II molecules, which can stimulate CD8 and CD4 T cells, respectively. Indirect Ag presentation occurs when EVs transfer Ag to antigen-presenting cells (APCs), which then present the Ag to T cells. Cross-dressing is a mechanism where p-MHC complexes from EVs are presented directly to T cells without reprocessing. EVs can also transfer native Ags to APCs, which can be processed and cross-presented to tumor-specific CTLs. EVs also serve as carriers of RNAs, which can be used for gene therapy. They can transport functional mRNAs and small non-coding RNAs, including miRNAs, which can regulate gene expression. EVs can also activate immune responses by delivering signals that promote the activation of acceptor cells into immunogenic APCs. Tumor-derived EVs can both stimulate and suppress immune responses. They contain tumor-specific Ags and immunosuppressive mediators such as FasL, TRAIL, and galectin-9, which can promote T-cell apoptosis. EVs derived from stem cells, such as mesenchymal stem cells (MSCs), have immunExtracellular vesicles (EVs), including exosomes, are small membrane vesicles derived from multivesicular bodies or the plasma membrane. They contain proteins, lipids, and nucleic acids from the parent cell and play a crucial role in intercellular communication by transferring their contents between cells. EVs are involved in various physiological processes and have important roles in immune regulation. EV-based therapeutics are being developed for inflammatory and autoimmune diseases and cancer. This review focuses on the role of EVs in modulating immune responses and their therapeutic applications. EVs are generated by inward budding of the endosomal membrane, leading to the formation of intra-luminal vesicles (ILVs) within endosomes, which are then termed multivesicular bodies (MVBs). MVBs can follow either the secretory or lysosomal pathway. In the secretory pathway, MVBs fuse with the cell membrane, releasing ILVs as exosomes. In the lysosomal pathway, MVBs fuse with lysosomes, releasing ILVs for degradation. The generation of exosomes requires sorting of exosome-targeted proteins and lipids into the endosomal membrane, delivery of the exosome cargo into nascent ILVs, and excision of ILVs. Monoubiquitylation of cytosolic domains of transmembrane proteins functions as a sorting signal to direct them to ILVs. The ubiquitylated proteins are captured by the endosomal sorting complex for transport (ESCRT) machinery. MVBs can still be generated in the absence of key subunits of all ESCRTs, indicating the existence of alternative mechanisms of exosome biogenesis. EVs can directly or indirectly present antigens (Ag) to T cells. Direct Ag presentation involves EVs carrying MHC Class-I and MHC Class-II molecules, which can stimulate CD8 and CD4 T cells, respectively. Indirect Ag presentation occurs when EVs transfer Ag to antigen-presenting cells (APCs), which then present the Ag to T cells. Cross-dressing is a mechanism where p-MHC complexes from EVs are presented directly to T cells without reprocessing. EVs can also transfer native Ags to APCs, which can be processed and cross-presented to tumor-specific CTLs. EVs also serve as carriers of RNAs, which can be used for gene therapy. They can transport functional mRNAs and small non-coding RNAs, including miRNAs, which can regulate gene expression. EVs can also activate immune responses by delivering signals that promote the activation of acceptor cells into immunogenic APCs. Tumor-derived EVs can both stimulate and suppress immune responses. They contain tumor-specific Ags and immunosuppressive mediators such as FasL, TRAIL, and galectin-9, which can promote T-cell apoptosis. EVs derived from stem cells, such as mesenchymal stem cells (MSCs), have immun