Rats injected with morphine exhibit decreased activity of the immune system, the bodily system that fights off infections  These same rats exhibited heightened blood levels of corticosteroids, chemicals secreted by the adrenal glands
Since corticosteroids can interfere with immune-system activity, scientists hypothesized that the way morphine reduces immune responses in rats is by stimulating the adrenal glands to secrete additional corticosteroids into the bloodstream
Removing the adrenal glands of rats, injecting the rats with morphine, and then testing the level of the rats' immune-system responses