The study by Raposo et al. investigates the secretion of antigen-presenting vesicles by B lymphocytes. They demonstrate that the limiting membrane of MHC II-enriched compartments (MIIICs) can fuse directly with the plasma membrane, releasing internal MHC II-containing vesicles, termed exosomes. These exosomes were isolated from cell culture media and found to differ significantly in surface protein composition from the plasma membrane. The exosomes contained compact, peptide-bound MHC II and were capable of presenting antigenic peptides to T cells, suggesting a role in antigen presentation in vivo. The study also discusses the potential physiological role of exosomes in the immune system, including their possible function as transport vehicles for MHC II- peptide complexes.The study by Raposo et al. investigates the secretion of antigen-presenting vesicles by B lymphocytes. They demonstrate that the limiting membrane of MHC II-enriched compartments (MIIICs) can fuse directly with the plasma membrane, releasing internal MHC II-containing vesicles, termed exosomes. These exosomes were isolated from cell culture media and found to differ significantly in surface protein composition from the plasma membrane. The exosomes contained compact, peptide-bound MHC II and were capable of presenting antigenic peptides to T cells, suggesting a role in antigen presentation in vivo. The study also discusses the potential physiological role of exosomes in the immune system, including their possible function as transport vehicles for MHC II- peptide complexes.