The immune system consists of complex cellular mechanisms that are controlled by stress-dependent, neuroendocrine, inflammatory and metabolic pathways to protect the organism from disease. To maintain adequate immune responses, a wide variety of immune cells from the innate (e.g. granulocytes) and the adaptive immune system (e.g. T-lymphocytes) [see figure] are orchestrated to fight against invading organisms and to control dormant virus load (e.g. Herpes). To monitor the overall function of cellular immune responses in vivo, the delayed type hypersensitivity (DTH) skin testing had been a valid standard measure but was phased out in 2002. In the meantime, an in vitro assay has been developed to allow assessment of antigen dependent immune responses triggered by T-lymphocytes. The change of the concentration of pro-and anti-inflammatory cytokines in the supernatant of the blood-antigen-mixture is hereby of particular interest to mirror the cellular immune reagibility. In this proposal, we suggest the implementation of this developed and improved diagnostic tool on the ISS to open up a new diagnostic approach to monitor functional, cellular immunity in space.