Humoral immune alterations caused by lead: studies on an adult toad model

there is evidence that environmental metal levels affect the immune function. in the particular case of the impact of heavy metals, information available suggests that the immune system is a target for low-dose pb exposure. among vertebrates it was shown that amphibians are capable of forming antibodies against a variety of antigens, causing several responses such as anaphylactic response and rejecting grafts. in this study, the production of antibodies was assessed against sheep red blood cells (srbc) in the anuran bufo arenarum after six weekly injections of sublethal doses of lead (50 mg.kg-1, as lead acetate). natural antibodies (natural heteroagglutinins) were also quantified against srbc. both assessments were carried out employing an elisa method developed to this end, measuring absorbance (a). for natural anti-srbc antibodies in both control (c) and pb treated (t) toads, there was a non significant tendency to increase the initial absorbances (c initial: 0.69±0.39 a; t initial: 0.54±0.30 a), relative to those registered at the end of the experiments (c final: 0.89±0.49 a; t final: 0.76±0.31a); the t/c ratios also did not show changes. the only significant difference was found between initial and final samples from lead-treated toads (p<0.014). the immune anti- srbc antibody levels of toads immunized with srbc showed a significant lower increase (p<0.05) in lead-treated animals (t final: 0.66±0.36 a), as compared to control toads (c final: 0.91±0.50 a) at the end of the experiment. it was thus concluded that the changes due to the assayed doses of pb in the levels of antibodies cannot be explained on the basis of only one single action mechanism of the metal, but as the result of a conjunction of effects over different immunocompetent cell subpopulations. these different responses suggest that factors affecting animals exposed to a foreign stimulus are different from those influencing the response of wild animals.