Immunotoxicology is the study of undesired modulation of the immune system by extrinsic factors. Toxicological assessments have demonstrated that the immune system is a target following exposure to a diverse group of
xenobiotics including ultraviolet radiation, chemical
pollutants,
therapeutics, and
recreational drugs. There is a well-established cause and effect relationship between suppression of the immune response and reduced resistance to
infections and certain types of
neoplasia. In humans, mild-to-moderate suppression of the immune response is linked to reduced resistance to common
community-acquired infections, whereas
opportunistic infections, which are very rare in the general population, are common in individuals with severe suppression.
Xenobiotic exposure may also result in unintended stimulation of immune function. Although a cause and effect relationship between unintended stimulation of the immune response and adverse consequences has yet to be established, evidence does suggest that
hypersensitivity, autoimmunity, and pathological
inflammation may be exacerbated in susceptible populations exposed to certain
xenobiotics.
Xenobiotics can act as
allergens and elicit
hypersensitivity responses, or they can modulate
hypersensitivity responses to other
allergens such as pollen or dust mite by acting as adjuvants, enhancing the development or expression of
hypersensitivity.
Allergic contact dermatitis,
allergic rhinitis, and
asthma are the most commonly encountered types of
hypersensitivity reactions resulting from chemical exposure. The immunologic effectors and mechanisms involved in autoimmune reactions are the same as those associated with responses to foreign
antigens; however, the reactions are directed against the host's own cells. Thus, chemicals that induce immune suppression, nonspecific immunostimulation, or
hypersensitivity may also impact autoimmunity. Risk assessment for immunotoxicity should be performed using the same approaches and principles for other noncancer effects. However, since
xenobiotics may have effects on more than one aspect of immune function, immunotoxicity data should be evaluated separately for evidence of suppression, stimulation,
hypersensitivity, and autoimmunity.