Xenobiotics natural defense systems

The immune system acts as the body s natural defense system to protect it from xeno-biotic chemicals infectious agents, such as viruses or bacteria and neoplastic cells, which give rise to cancerous tissue. Adverse effects on the body s immune system are being increasingly recognized as important consequences of exposure to dangerous species. Pollutants can cause immunosuppression, which is the impairment of the body s natural defense mechanisms. Xenobiotics can also cause the immune system to lose its ability to control cell proliferation, resulting in leukemia or lymphoma. 

Unfortunately, the activity of the natural defense system weakens with age. At the same time, more frequent are situations that enhance in vivo oxidation. For example, stress, excessive physical effort and numerous external factors, such as pollution (xenobiotics generate radicals), ionizing radiation, excessive availability of transition metals, redox cycling drugs, and tobacco, all play a role as well. 

For a number of overt, broad spectrum immunosuppressive xenobiotics (e.g., azathioprine) there is sufficient clinical experience to indicate the types of neoplasms for which there is an increased risk. These tumor types are listed in Table 27.1. Also listed are the tumors that occur in the unfortunate experiment of nature, namely patients infected with human immunodeficiency virus type 1 (HIV-1) and the tumors that may occur at higher incidence with more selective yet strong immunosuppressants (e.g., cyclosporin, sirolimus, and tacrolimus). Compared to the broad spectrum immunosuppressive agents listed above, most IMBPs express a highly selective regulatory influence on the immune system modulating the activity of host defense systems rather than mediating frank immunosuppression. 

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