The effect of space flight on human cellular immunity.

T-lymphocyte responsiveness to mitogens is depressed by an average of 56% in humans (129 subjects) tested during and after space flight. Although there is not yet conclusive evidence of a clinical significance of the test, it is clear that factors of space flight like stress, closed environment and cosmic radiation may affect immune responsiveness. The data obtained from space crews may be compared to the depression seen in subjects undergoing heavy physical stress of head down tilt bedrest. Recently, delayed hypersensitivity [correction of hypersensivity] response was tested on crews of the US space shuttle and of the orbital station MIR by means of a commercially available "skin test". Again, the response was lower in 14 of the 15 subjects tested. In two cases, a strong in flight depression could be related to heavy physical and psychological stress experienced in flight. The data available today are not sufficient to draw conclusions on the hazard of infectious diseases during and after space flight. Although the changes observed never harmed the health of astronauts, immunological changes must be seriously investigated and understood in view of long-duration flights on space stations in an Earth orbit and to other planets like Mars and the Moon.