A recent study by David Denhardt and team, from Rutgers University, suggests that weightlessness may be bad for the immune system. The study showed that simulated weightlessness caused the spleen and thymus organs to decrease as much as 70% in size. These organs are responsible for triggering the creation of white blood cells in the body. White blood cells in turn destroy diseased cells and invaders that are in the blood stream. The decrease in size of the spleen and thymus organs was connected to an increase in the hormone called osteopontin (OPN).
The weightlessness was simulated by lifting up the mice's hind legs. I'm not sure how this was decided to be an accurate depiction of the effects of weightlessness - it seems a little fishy to me. But perhaps there is tons of science behind the method. For instance, Osteopontin has been connected with the bone loss that occurs in space.
None-the-less this study represents a significant setback for Heinlein fans who were really hoping that the moon could be turned into the best retirement village in the world (after we get rid of the giant, killer bugs).