Exosomes are small membrane vesicles, secreted by most cell types from 
multivesicular endosomes, and thought to play important roles in intercellular 
communications. Initially described in 1983, as specifically secreted by 
reticulocytes, exosomes became of interest for immunologists in 1996, when they 
were proposed to play a role in antigen presentation. More recently, the finding 
that exosomes carry genetic materials, mRNA and miRNA, has been a major 
breakthrough in the field, unveiling their capacity to vehicle genetic messages. 
It is now clear that not only immune cells but probably all cell types are able 
to secrete exosomes: their range of possible functions expands well beyond 
immunology to neurobiology, stem cell and tumor biology, and their use in 
clinical applications as biomarkers or as therapeutic tools is an extensive area 
of research. Despite intensive efforts to understand their functions, two issues 
remain to be solved in the future: (i) what are the physiological function(s) of 
exosomes in vivo and (ii) what are the relative contributions of exosomes and of 
other secreted membrane vesicles in these proposed functions? Here, we will 
focus on the current ideas on exosomes and immune responses, but also on their 
mechanisms of secretion and the use of this knowledge to elucidate the latter 
issue.