Exosomes are small vesicles that are secreted from cells. They are derived from multivesicular endosomes that fuse with the plasma membrane, thereby releasing their internal vesicles into the extracellular environment. Exosomes from antigen-presenting cells contain a range of immunostimulatory molecules that activate T cells, which suggests that they may have an important role in the propagation of immune responses. Of considerable interest is the finding that exosomes derived from bacterially infected macrophages carry bacterial coat components and use these to stimulate bystander macrophages and neutrophils to secrete proinflammatory mediators, including tumor necrosis factor-alpha, the chemokine regulated upon activation, normal T cell-expressed and-secreted (RANTES, also known as CCL5), and inducible nitric oxide synthase. Here, we address these studies in relation to other findings on dendritic cell-derived exosomes that are also powerful immunoregulators.