Are pesticides immunotoxic?

So far there is little evidence that occupational or environmental exposure to pesticides has led to clinically significant immunosuppression, and hence to an increased risk of developing infection or cancer. In addition, the incidence of hypersensitivity reactions to pesticides is generally low. Experiments have been conducted in experimental models that indicate that certain pesticides are immunosuppressive to animals. The majority of these experiments, however, have used high (frankly toxic) doses of pesticides and immunosuppression has been monitored using in vivo or in vitro immune function tests, the results of which are difficult to interpret in terms of effects on health. One exception is tributyltin oxide which, in the rat, causes immune dysfunction at doses below those that cause general toxicity, and which compromises the ability of the animals to resist bacterial and parasitic infection. Predictive assessment of possible immunotoxicity induced by exposure to a pesticide should be structured within the current framework of acute, subacute and chronic testing procedures used for regulatory purposes. With the exception of predicting some hypersensitivity reactions (respiratory allergy and autoimmunity), which would require the development of novel specialized methods, indications of potential immunotoxicity can be obtained from standard haematological investigations and by evaluation of lymphoid organs and tissues such as the spleen, thymus, lymph nodes, and bone marrow. Pathological and histopathological examination of the lymphoid system is a mandatory requirement of nearly all subchronic testing guidelines for pesticides worldwide. The incorporation of specialized, and in particular in vitro, immune function tests into the routine toxicological assessment of a pesticide is not only time-consuming and potentially wasteful of animals, but is also scientifically unacceptable; the significance of changes in such tests must await further research on the reserve capacity of the immune system.