Abstract: OBJECTIVE：To investigate immunotoxicity of the main by-product 2-acetyl-4-tetrahydroxybutylimidazole (THI)，in the process of class Ⅲ caramel color production，on female rats. METHODS： 50 healthy adult female SD rats with SPF grade were randomly divided into 5 groups，10 rats in each group. They were given THI orally at doses of 0，0.1，0.5，2.5 and 5 mg/kg，respectively，continuously for 30 days. The followings were collected/evaluated：body weights，organ coefficients of thymus and spleen，histopathological evaluation of thymuses， number of white blood cells and various types of cells in peripheral blood， proliferation ability of spleen T lymphocytes，activities of NK cells，and flow cytometry evaluation of changes in the proportion of major immune cells in peripheral blood，thymus and spleen. RESULTS：THI exposure reduced the area ratio of thymus cortex/medulum and the proliferation ability of rat spleen T lymphocytes (with 2.5 and 5 mg/kg doses). The results of immunocytopathology showed that THI exposure at a dose of 2.5 mg/kg and above reduced the ratio of the total number of white blood cells and T lymphocytes in the peripheral blood and increase the ratio of NK cells in the spleen. The number of lymphocytes and the ratio of B lymphocytes in the peripheral blood were significantly reduced，and the ratio of T cells in the spleen was also reduced by the 5 mg/kg THI exposure. For the T cell subsets，5 mg/kg THI exposure reduced the ratio of Th cells in the peripheral blood. With doses of 0.5 mg/kg and above，THI increased the ratio of Th cells in the thymus and CD4⁺CD8⁺ double positive. The proportions of non-positive T cells were reduced. With doses of 2.5 mg/kg and above，THI increased the proportion of CTL cells in the thymus. CONCLUSION：Oral exposure to THI caused damage to the immune system of female rats. In addition， changes in the number of lymphocytes in peripheral blood and the proportion of thymus T cell subsets were sensitive indicators. Thus， the no observed adverse effect level of THI immunotoxicity (NOAEL) is 0.1 mg/kg.

Key words: 2-acetyl-4-tetrahydroxybutyl imidazole, rat, immunotoxicity, lymphopenia