This paper first summarizes the studies indicating that the immunotoxi
c effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), notably atrop
hy of the thymus and suppression of the thymus-dependent immunity, are
mediated by binding to a soluble cytosolic protein, the aryl hydrocar
bon (Ah) or TCDD receptor, present in the thymus in the epithelial cel
ls. On the basis of a common receptor-mediated mechanism of toxic acti
on, the relative (immuno)toxicity of individual PCDDs and PCDFs can be
expressed relative to TCDD (i.e., toxic equivalents). Next, studies o
n TCDD-induced immunosuppression and impaired host resistance, and low
est observed effects levels of TCDD resulting in immune alterations, a
re summarized. Immune investigations performed in man are discussed an
d it is concluded that, for risk assessment purposes, further studies
are necessary to determine the sensitivity of the human immune system
to TCDD. For this purpose, a recent study is summarized in which the s
ensitivity of the human thymus to TCDD is investigated in so-called se
vere combined immunodeficient (SCID) mice in which human thymus grafts
were transplanted. This study indicates that the human thymus and the
Wistar rat thymus display a similar sensitivity to TCDD. (C) 1998 Wil
ey-Liss, Inc.