Natural Killer Cells and Immunotoxicology

Immunotoxicology can be simply defined as the study of adverse effects to the immune system. However, because of the complexity of the normal immune system, adverse effects by a xenobiotic can alter the system in a variety of ways: suppression, upregulation as in the case of xenobiotic-induced autoim-munity or hypersensitivity, or specific alterations of certain types of cells. During the last 20 yr, many observations have been made concerning chemical or drug-induced immunosuppression (1 –3 ). Examples of these include polyhalogenated aromatic hydrocarbons such as TCDD, polycyclic aromatic hydrocarbons such as benzo(a)pyrene, and drugs (opioids, cannabinoids, and lithium). Typically, alterations to the immune system are manifested in decreased host resistance. The immune system works in a concerted way to prevent invasion by a foreign substance. Therefore, when the system is damaged, either the defenses break down so that the foreign agent can enter the body and cause disease, or the immune system itself may attack the body as in the case of hypersensitivity.