Evaluation of Immunotoxicity Induced by Organophosphorus Pesticide Malathion

Malathion (MLT) is an organophosphate pesticide widely used worldwide. Due to its environmental persistence and accumulation in living organisms, concerns have been raised regarding its potential health effects beyond the classical mechanism of cholinergic inhibition, particularly its impact on immune function. In this study, we aimed to systematically evaluate the immunotoxicity of MLT in mice and identify the lowest-observed-adverse-effect level (LOAEL) for immunotoxic effects. Key parameters assessed included body and organ weights, hematological and clinical chemistry profiles, histopathological changes, and immune function indicators. The results showed that exposure to MLT, particularly at low and intermediate doses, led to a significant increase in thymus weight, along with marked reductions in interleukin-10 (IL-10) levels, neutrophils, polychromatic erythroblasts, and monocyte lineage cells. Histological examination revealed atrophy of splenic white pulp, indicating immunopathological alterations predominantly at these dose levels. In contrast, immunoglobulin G (IgG) levels increased in a dose-dependent manner, possibly reflecting a compensatory humoral response to the observed suppression of cellular immune components. Meanwhile, the plaque-forming cell (PFC) response exhibited a dose-dependent trend but was significantly inhibited only at the highest dose, suggesting a complex, non-linear effect on humoral immunity. Based on significant alterations in thymus weight, cellular immune parameters, and splenic histopathology observed at the lowest dose tested (16 mg/kg bw), this value was preliminarily identified as the LOAEL for MLT-induced immunotoxicity in mice.