in experimentally exposed animals 2,3,7,8-tetrachlorodibenzo - n - dioxin ( tcdd ) causes severe immunosuppression . 
 however , the overall susceptibility of humans for the different pathological effects of tcdd has remained unclear . 
 we examined the long - term effects of tcdd in 11 industrial workers who were exposed to high doses of tcdd for several years 20 years ago . 
 current tcdd body burdens were still at least 10 times higher ( between 43 and 874 pg / g blood far ) in these exposed persons than in the average german population . to evaluate possible tcdd - induced changes in the percentage of different lymphocyte subsets , we determined a large panel of lymphocyte subsets in the blood by flow cytometric analysis . 
 immunocompetence of t - and b - lymphocytes was tested by nitrogen ( phytohemagglutinin , pokeweed mitogen)- induced lymphoproliferation assays and by assays using sensitive mixed - lymphocyte cultures . 
 no significant differences could be detected between the individuals tested and controls for surface marker distribution or mitogen - induced lymphoproliferation tcdd - exposed subjects showed a reduced response to human lymphocyte antigen - allogeneic lymphocytes and interleukin-2-boosted proliferation . 
 responder cells of the dioxin - exposed persons proliferated less in response to irradiated stimulator cells ( p < or = 0.05 ) , and the third - party mixed lymphocyte reaction against unirradiated stimulator cells revealed suppressive activity in the responder cell fraction compared to the controls ( p < or = 0.01 ) . 
 furthermore , the capacity of a pool of t cells isolated from tcdd - exposed subjects to proliferate upon interleukin-2 stimulation was significantly diminished ( p < or = 0.05 ) . 
 tcdd has a long - term immunosuppressive - effect on t - helper cell function , which is mediated more likely by a reduced functionality of individual cells rather than by a reduction in absolute cell numbers in the peripheral blood.imagesfigure 1 . 
 afigure 1 . 
 bfigure 2.figure 2 .