Opioids are known to affect the immune system and their use often results in harmful consequences. For example, higher than normal rates of infections such as skin abscesses, thrombophlebitis, septicemia, bacterial endocarditis, tetanus, and malaria have been observed in narcotic addicts (Hussey & Katz, 1950). Other studies have also reported hepatitis as being one of the most frequently observed infections among opioid addicts, as well as pulmonary infections and cellulitis (Louria et al., 1967). High elevations of immunoglobulin M (IgM) in heroin addicts have been shown to occur (Brown et al., 1974), which also occurs in hepatitis patients, and may reflect an increased susceptibility to infection. Decreases in the response of T-lymphocytes to phytohemagglutinin (PHA) have been observed in both heroin addicts and in subjects undergoing 1- to 5-day withdrawal from heroin compared to controls (Govitrapong et al., 1998). Whether these complications are due to the direct effects of opioids, impurities in these street drugs, or are a consequence of the careless habits of drug addicts such as the sharing of needles, remains to be determined. There are a limited number of human studies that have been able to attribute immune alterations to the direct effect of acute opioid administration. For example, morphine administration in healthy volunteers was found to depress peripheral blood spontaneous natural killer cell cytotoxicity (Yeager et al., 1995). It is, however, difficult to study opiate influences on the immune system in humans due to ethical issues and a lack of control over variables. Due to these factors, the effects of opioids on the immune system can be more easily studied using animal models because the experimenter has more control over the experimental conditions.
             Opiates and the Immune System: Animal Models
             In animal models, the effects of opiates on the immune system are complex, and depend on drug dose, frequency of ad...