There is no doubt that TCDD is capable of inducing effects on a variety of components and functions of the immune system in a variety of species. In fact, such changes seem to belong to the most sensitive variables affected by TCDD. Some of the biological effects, induced at rather high doses of TCDD exhibiting general toxicity (> 3 micrograms TCDD/kg body wt), may be considered unspecific or the result of the pronounced thymus involution. However, other effects (such as that on lymphocyte subtype patterns in marmosets or a reduced resistance of mice to influenza viruses) have been reported to occur at dose levels far from those leading to thymic involution or general toxicity. It should be remembered that the pathognomonic relevance for man of subtle modifications in the pattern of lymphocyte surface receptors is largely unknown. Until now, such deviations are considered rather as biological phenomena than indications or causes of specific diseases. Nevertheless, such changes represent clear-cut biological effects induced by TCDD. Since effects of TCDD on components and defined functions of the immune system have been revealed in several species, it would be surprising if humans were largely resistant to such effects, but reliable data in humans with high exposures to defined dioxins verified by an appropriate quantification of the exposure are scarce as of now. Data published so far have not revealed pronounced alterations of such variables. However, no studies of well-defined human populations with quantified body burdens have been performed with modern methods (such as flow cytometry) analyzing a wide variety of surface receptors. Performance of such studies is essential for a better and reliable risk assessment, and the technology is available. Some of the effects observed (such as the changes in the pattern of lymphocyte subpopulations) must certainly be considered as biological effects induced by TCDD, and the situation is similar to the induction of hepatic monooxygenases, which are also observable in this dose range. However, the relevance of such changes with respect to adverse health effects in humans is presently difficult to judge in the absence of clear-cut functional deficits demonstrated so far either in vivo or in vitro.
