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The toxic effect of paraquat (PQ), an herbicide that has been used widely in agriculture, on some parameters of the immune system was investigated. PQ was administered to Balb/c mice as intraperitoneal doses of 0.01, 0.1, or 1.0 mg PQ/kg body weight (BW) for a total of 28 days. Besides the histopathological examination of each host?s vital organs, measures of their splenic and bone marrow cellularity, blood macrophage/granulocyte phagocytic activity, total serum immunoglobulin M (IgM) and IgG, complement C3, and splenic CD49b (natural killer) cell levels, as well as of serum aspartate aminotransferase, alanine aminotransferase, and lactate dehydrogenase and complement-mediated lytic activity (CH₅₀), activities, were then performed following the respective final treatments. The results indicate that high and medium doses of PQ (i.e., 1 and 0.1 mg/kg) could lead to suppressed phagocytic activity by circulating macrophages/granulocytes. The data also revealed that the high PQ dose induced a significant decrease (p<0.05) in spleen cellularity and splenic CD49b cell levels, along with numerous histopathological changes in the spleen. However, at none of the doses tested did PQ produce changes in serum levels of C3, total IgG or IgM, or in the CH₅₀. At 0.01 mg PQ/kg/day, no histopathological or functional disturbances were detected. These results indicate that PQ at doses more than 0.1?mg/kg has toxic effects on the cellular components of the innate immune system of Balb/c mice. The present results, however, indicate that at an exposure level below the recommended acceptable daily intake limit of 0.005 mg/kg, no observable immunotoxicity effect might be expected.

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