The impact of smoked marijuana on immune function remains a contro- versial topic, particularly given recent medicinal use of the drug. Many people employ cannabis in the battle against AIDS-related wasting and the anorexia associated with cancer chemotherapy. Physicians and pa- tients show an understandable concern about any treatment that might impair the ability to ward off illness. People with AIDS and cancer need all their immune cells to function as well as possible. Research on THC and immunity has focused on individual cells, live animals, and humans. Generally, the impact of the drug is most dramatic on small cultures of cells and has less effect on immunity in people.
The vast majority of research on marijuana and immunity focuses on the white blood corpuscles (leukocytes or lymphocytes) known as T-cells. The thymus gland helps these leukocytes develop, hence, the name “T” cell. These lymphocytes can bind to cells that have been infected with a virus. The T-cells then release molecules that will destroy the infected cell. This process not only eliminates the site of the infection, but it also prevents the spread of the virus. Thus, these T-cells play an important role in immune function.
One initial study found that T-cells taken from chronic marijuana smokers were less responsive to immune system stimulants than the
T-cells taken from nonsmokers (Nahas, Suciv-Foca, Armand, & Morish- ima, 1974). Yet this finding has not replicated in other work. Many stud- ies have shown no immunity problems in cells taken from marijuana smokers (IOM, 1999). Another research approach involves extracting human cells from nonsmokers and exposing the cells to large amounts of THC or cannabis smoke. This treatment decreases the cell’s responses to chemicals that normally stimulate immune function. Yet heavy doses of caffeine, aspirin, and alcohol show comparable effects. Thus, these data do not justify alarm about marijuana’s potential impact on immunity.
Another intriguing line of research focuses on alveolar macrophages, the immune cells that clear infectious organisms from the lungs. Re- searchers used a saline wash of the lungs to remove some macrophages from people who smoked marijuana regularly. Compared to the cells from nonsmokers, the macrophages extracted from marijuana smokers were less able to kill fungi, bacteria, or tumor cells. Although the behav- ior of extracted cells may not generalize to the immune function of hu- mans, these data support the idea that heavy marijuana smoking could lead to pulmonary problems by inhibiting the immune cells of the lungs (Tashkin, 1999). Nevertheless, this sort of treatment of isolated cells may reveal little about the way that they might function inside a human. Thus, the relevance of this work to real people catching colds, flu, or other illness may be limited (Zimmer & Morgan, 1997).
Research from living animals might prove more relevant than findings from extracted cells. Studies of guinea pigs and mice reveal that THC can reduce their ability to ward off viruses and bacteria. For example, guinea pigs given THC and then exposed to the herpes virus showed more symptoms than those guinea pigs who do not receive THC. Mice treated with THC had less resistance to other viruses and bacterial in- fections, too (Cabral, 1999). Yet some of these studies use enormous doses of THC, as high as 200 mg per kilogram of body weight. An equal amount, based on weight, for a 160-pound person, would require smok- ing more than 700 joints. This dosage obviously surpasses the consump- tion of even the most devoted cannabis users. Critics of this research emphasize that these doses are unrealistic analogues of human consump- tion.
Given the difficulties inherent in generalizing from animal research, studies of humans may provide the most relevant evidence for THC’s impact on immune function. An ideal scientific approach would ran- domly assign people to smoke marijuana or a placebo for months and
months, and then measure which group gets sick more often. Obvious ethical problems have prevented research of this type. Instead, studies focus on the rates of illness in people who choose to use cannabis and in those who do not. This type of work must control for differences be- tween these groups in lifestyle habits and the use of other drugs. These studies are rare. One shows a small increase in respiratory symptoms in cannabis smokers (Polen, 1993). Comparable studies of HIV-positive people found that smoking marijuana had no impact on their immune function (Coates et al., 1990; Kaslow et al., 1989). Another study ac- tually revealed that heterosexual men who used cannabis regularly were less likely to develop non-Hodgkin’s lymphoma (Holly, Lele, Bracci, & McGrath, 1999). Dronabinol, the synthetic version of THC prescribed for oral consumption, does not come with warnings about potential loss of immune function. The Institute of Medicine has concluded that the impact of marijuana smoke itself requires further research (IOM, 1999). Given results like these, THC does not appear to impair immunity, and limited ingestion of marijuana smoke may also have little effect.
Conclusions
A great deal of research addresses the role of cannabis on health. Mari- juana does not appear to have a toxic dose. The drug can exacerbate symptoms of some mental illnesses, particularly psychotic disorders like schizophrenia. Yet it does not appear to cause these mental health prob- lems. Cannabis’s impact on brain structure is minimal. Nevertheless, sen- sitive measures of brain function reveal subtle changes associated with years of regular use. Respiratory symptoms like bronchitis and wheezing appear more often in chronic cannabis users; they also show changes in bronchial cells comparable to those seen in early stages of lung cancer. High doses of cannabinoids can alter sperm production and reproductive hormones, but these effects are temporary. The impact of THC and can- nabis smoke on immune function may require further investigation, but data have yet to show that smoking marijuana increases the rates of in- fectious disease.
These results confirm that marijuana is neither completely harmless nor tragically toxic. Compared to other drugs that are currently legal, its impact on health is minimal. People with psychotic disorders should probably avoid cannabis. Chronic daily use obviously creates potential
problems for the quick performance of complex tasks. Smoking every day undoubtedly taxes the lungs. Men attempting to impregnate women may have more luck if they abstain from cannabis. Pregnant women should probably avoid all drugs. Nevertheless, occasional use by healthy adults does not appear to create dramatic mental or physical illness. Can- nabis seems to have fewer negative health effects than legal drugs, like alcohol, caffeine, or tobacco, and kills far fewer people.
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