tential damage: the immune system, the cardiovascular system, and reproductive and fetal health.

As discussed in the last chapter, several biological studies suggest that cannabinoids can depress the immune system's response to infection. In some experiments, white blood cells in experimental animals exposed to THC and other cannabinoids exhibited a reduced capacity to proliferate following infection; some animals also produced fewer than normal antibodies or showed signs of impaired “killer cell” activity.7

Not all studies of this nature implicate cannabinoids as immune suppressants. In fact, some immune functions have been found to increase in response to cannabinoids. Such results are not necessarily contradictory because many physiological processes contribute to immunity. Thus, no single experiment can truly reveal the “big picture” of marijuana's effects on the human immune system. That is particularly true of studies that test the effects of pure cannabinoids such as THC, since marijuana contains a variety of chemicals that may affect immune activity.

Although it demands equally cautious interpretation as studies on individual immune cells, research on disease resistance in animals exposed to cannabinoids more closely tracks the overall impact of cannabinoids on the immune system. Mice infected with pneumonia-causing bacteria died of septic shock when they were injected with THC before and after infection; those that did not receive THC developed immunity to the bacterium and survived. This response was found to vary depending on the amount of THC the mice received and whether it was injected before or after they were infected with the bacteria. Similarly, two doses of THC given before and after infection with the herpes simplex virus appeared to hasten the death of immunodeficient mice, although a single dose of THC prior to infection did not. Both experiments suggest that the timing of THC exposure relative to infection determines whether THC suppresses the immune response.8

Even if cannabinoids themselves cause little or no harm to the immune system, there is good reason to believe that smoking marijuana does. Marijuana smoke been linked to increased mortality in people with AIDS and it also appears to injure an important class of immune cells in the lungs. These cells, called alveolar



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