Immune responses protect the body from disease through the development of various “defense cells,” such as antibodies, macrophages, and T cells. Research of the immunobiological effects of cannabis is widely contradictory. Biased researchers have often interpreted ambiguous evidence according to preconceived theories. Test-tube studies finding cellular suppression of immune responses have been flawed by their use of extremely high concentrations of cannabinoids, levels impossible to obtain in actual use. The same flaw is found In research cited as proof that cannabinoids impair cellular metabolism. Misinformed persons might consider that AIDS patients are a high-risk group for possible immune complications brought about by the use of medical marijuana, but there is no clinical or epidemiological evidence linking cannabis use to immune suppression. According to cannabis researcher Leo Hollister,
Clinically, one might assume that sustained impairment of cell-mediated immunity might lead to an increased prevalence of malignancy. No such clinical evidence has been discoved or has any direct epidemiological data incriminated marijuana use with the acquisition of human immunodeficiency virus or the clinical development of AIDS.
Clinical evidence of immune suppression is somewhat contradictory. For example, N.E. Kaminsky stated in the Journal of Neuroimmunology, “I think that these (cannabinoids) might be useful as a relatively weak immune modulators, perhaps to be used as anti-inflammatory agents or even maybe for asthma.”  (Asthma is thought to be an autoimmune disease).
Modern understandings of the endocannabinoid systems within the human body show that cannabinoids are actually building blocks of our immune systems. Dr. Donald Abrams, one of the foremost authorities on cannabis in the treatment of HIV and AIDS reports:
“ The CB2 receptor, interestingly, the second cannabinoid receprot, is not found in the brain at all, but is predominantly located in the immune issues, the macrophages which are circulating blood cells that go into tissues and fight infections, and also the spleen.” 
Research also indicates that the tolerance factor found in most aspects of cannabis use probably protects the subject from any potential immunological dangers. Immune suppression that may be caused by a large overdose of cannabinoids has shown to be reduced by repeated exposure.
Several human studies of large cannabis-using populations show no difference in disease susceptibility between cannabis users and nonusers.  According to the United Nations World Health Organization in 1997,
To date there has been no epidemiological evidence of increased rates of disease among heavy cannabis users. Given the duration of large scale cannabis use by young adults in Western societies, the absence of any epidemics of infectious disease makes it unlikely that cannabis smoking produces major impairments in the immune system.
It is possible that a small percentage of users may develop allergies to cannabinoids with repeated exposure. It is also possible that the rarely documented cases of allergic reactions to smoked cannabis have been caused by other factors such as fungi, mold, toxic insecticides, and/or herbicides. Because of the possibility of exposure to contaminant hazards, AIDS sufferers and patients with other immune deficiencies should not obtain cannabis through illegal non-medical markets.
Related sections: Cardiovascular Effects, Contaminants, Smoking Methods, Tolerance, Upper Respiratory Infection.
 Hollister, “Health aspects of marijuana.” Pharmacological Review, Vol. 38, No. 1, 1986
 Hollister, “Marijuana and immunity.” Journal of Psychoactive Drugs, Vol. 24, pp. 159-63, 1992
 World Health Organization Project on Health, Implications of Cannabis Use, 1997
 “The health and psychological consequences of cannabis use.” Chapter 6, National Drug Strategy Monograph No. 25, Australia
 Hollister, op. cit.
 Finn, “Cannabinoid investigations entering the mainstream.” The Scientist, Vol. 12, No. 3, pp. 1-8, February 2, 1998
 Abrams. Lecture, Lindesmith Center, San Francisco, May 17, 1999
 Loveless, Harris, and Munson, “Hyporesponsiveness to the immunosuppressant effects of delta-8 tetrahydrocannabinol.” Journal of Immunopharmacology, Vol. 3, No. 3-4, pp. 371-383, 1981
 World Health Organization, op. cit.