The immune system is equipped with well-regulated mechanisms of defense against invading pathogens or any agents that are recognized as nonself. Natural barriers and antigen-nonspecific reactions of cells such as granulocytes and macrophages provide direct and first line of response to the foreign antigens. The second line of defense consists of cells that through fetal-neonatal “education” eliminate nonself antigens. Immune reactions that are antigen nonspecific as well as those that depend on recognition of specific antigenic epitopes can be compromised by xenobiotics encountered in occupational and nonoccupational settings. The immune system can be affected by chemicals to which we are exposed through inhalation, by food consumption, or by skin contact. A growing number of allergies, development of autoimmune diseases, and compromised defense mechanisms to fight off infections could result from this exposure to toxicants. Depending on the exposure route, different...