Exosomes are cellular microvesicles, which have already demonstrated their potential to induce specific immune responses. The majority of permanent cell lines spontaneously release exosomes into the supernatant, from where they can be easily purified and concentrated [30] . Transfecting HEK293 cells with expression plasmids coding for CD154 and gp350 resulted in the secretion of modified exosomes carrying both proteins. Upon interaction with their target B cells, an undetermined fraction of exosomes is presumably engulfed and degraded in lysosomes in such way that peptides of exosomal proteins are presented in association with MHC class II molecules. Exosomes that are not taken up by the target cells probably engage in protein-protein contacts with cellular surface molecules and lead to receptor activation and signaling. gp350 molecules on the surface of exosomes probably target these vesicles exclusively to human B cells which express relevant amounts of the receptor molecule for gp350, CD21. Exosomal co-transfer of both gp350 and CD154 is thought to lead to efficient CD154/CD40 ligand/receptor engagement on the B-cell surface activating the intrinsic CD40 signaling cascade.