http://www.ukcia.org/research/EffectsOfMarijuanaOnLungAndImmuneDefenses.php
Effects of Marijuana on the Immune System
In vitro and animal studies
The recent finding of cannabinoid receptors (to which THC binds) on white blood cells (Bouaboula et al.1993) is consistent with observations that THC is capable of influencing immune responses. In vitro and animal studies suggest that THC has a general immunosuppressive effect on a variety of immune cells, induding rnacrophages, natural killer cells, and T cells (Burnette Curley and Cabral 1995; Huber et al. 1975, 1980; Klein et al. 1991; Kusher et al. 1994). Mice exposed to D9THC were unable to develop protective immunity against lung infection by Legionella pneumophilia, an opportunistic pathogen (Newton et al. 1994).
Immune deficits in marijuana smokers
As noted above, alveolar macrophages from the lungs of healthy, habitual marijuana smokers were suppressed in their ability to kill fungaland bacterial organisms, as well as tumor cells. Moreover, the same cells were suppressed in their ability to release proinflarnmatory cytokines. These findings suggest that marijuana is an immunosuppressant with clinically significant effects on host defense, which could have potentially serious health consequences in patients with preexisting immune deficits due to AIDS, organ transplantation (receiving immunosuppressive therapy to prevent rejection of the transplant), or cancer (receiving immunosuppressive chemotherapy). The latter possibility is supported by reports of fungal and bacterial pneumonias in patients with AIDS or organ transplantation who used marijuana (Caiaffa et al. 1994; Denning et al. 1991). Moreover, among HIV-positive individuals, active marijuana use has been found to be a significant risk factor for rapid progression from HIV infection toAIDS and acquisition of opportunistic infections and/or Kaposi's sarcoma (Tindall et al. 1988).