Natural killer (NK) cells contribute to immunosurveillance and first-line defense in the control of tumor growth and metastasis diffusion. NKEVs are constitutively secreted, are biologically active, reflect the protein and genetic repertoire of their originating cells and exert anti-tumor activity in vitro and in vivo. NKEVs from tumor-conditioned NK cells interact with naïve NK cells promoting their cytotoxic activity. In cancer NK cells exhibit profound defects in degranulation ability, a status probably reflected by their NKEVs. Hence, NKEVs could contribute to improve cancer therapy by interacting with tumor and/or immune cells at the same time sensing the actual NK cell status in cancer patients. Here we investigated the role of NKEVs in stimulating the immune system and developed an immune enzymatic test (NKExoELISA) to sense the systemic NK cell status by measuring plasma NK-derived exosomes through combined capture of exosomes, expressing typical EV (tsg101) and NK cell (CD56) markers. We analyzed by LC-MS/MS the protein content from NKEVs evaluating proteins differentially expressed in exosomes (NKExo), vescicles (NKMV) and total cell extract (Tot extr) from parental NK cells. Proteomic data confirmed the presence of many EV markers and detected several proteins involved in immune response, cell adhesion and complement biological processes.