Exosomes are small membrane-bound vesicles that in recent years have emerged as important molecules for inter-cellular communication. Exosomes are produced during both normal and patho/physiological conditions, and cancer cells have been shown to secrete exosomes in greater amounts than normal cells. CD81 is a 26 kD non-glycosylated member of the tetraspanin superfamily (TM4SF),which is expressed on T and B cells, NK cells, monocytes, dendritic cells, thymocytes, endothelial cells, and fibroblasts.CD81 associates with several other cell-surface proteins in a multimolecular complex, including CD19, CD21, CD20, CD37, CD53, and CD82 in B cells, and CD4, CD8, and CD82 in T cells. CD81 is frequently expressed on exosome membranes used to characterize exosome populations from a variety of body fluids.