Abstract： Objective To investigate the effect of nonspecific immune reaction mediated by macrophages on brain injury induced by acute carbon monoxide (CO) poisoning in rats.Methods Twenty-four male healthy rats were randomly divided into the 3 days after CO poisoning group (or group A),the 7 days after CO poisoning group (or group B) and the normal control group (or group C),each consisting of 8 animals.Models of acute CO poisoning were established by using the animals in group A and group B.Samples of the brain tissue were collected following abdominal anesthesia.Electron microscopy,immunohistochemistry and Western blot were used to observe changes in ultra-structure of the brain tissue,infiltration of macrophages and the expression of related cytokines.Results Following acute CO poisoning,neuronal degeneration,necrosis and microglia accumulation around the injured neurons could be observed under electron microscopy.Macrophage infiltration and expressions of macrophage inflammatory protein-1α (MIP-1 α) and intercellular adhesion molecule-1 (ICAM-1) were observed in the brain of rats,while for the animals in the control group,there was little expression of macrophage specific protein (ED-1).The No.of positive cells in the cortex was 2.83 ± 1.72,and the No.of positive cells in the hippocampus was 2.33 ± 1.37.The peak of macrophage infiltration was seen 7 days after CO poisoning,with the No.of positive cells in the cortex being 30.33 ± 1.14 and the No.of positive cells in the hippocampus being 16.00 ± 1.41.Statistical significance could be seen,as compared with those of the control group (P ＜ 0.05).However,there was little the expression of MIP-1 α in the control group.Under the electron microscope,the No.of positive cells in the cortex was 5.83 ± 2.48 and the No.of positive cells in the hippocampus was 6.67 ±2.94.The expression of MIP-1α for group A was most significant,with the No.of positive cells in the cortex being 238.33 ± 11.94 and the No.of positive cells in the hippocampus being 207.83 ± 9.79.Statistical significance could be noted,when comparisons were made with the control group(P ＜0.01).The trend in expression of ICAM-1 was identical to that of MIP-1α,but little expression of ICAM-1 could be seen in the control group,with IOD in total brain being 10.56 ± 16.11.The expression for group A was most significant,with IOD in total brain being 845.77 ± 80.70.And statistical significance could be noted,when comparisons were made with the control group(P ＜0.01).Conclusions Nonspecific immune reaction mediated by macrophages was involved in brain injury,following acute CO poisoning,and the reaction might be associated with the chemotactic effect of cytokines.