Abstract： Objective To explore the possible role of CD28 +/CD152 +:B7 eostimulators in immune pathophysiology of severe pneumonia.Methods 22 severe pneumonia peripheral blood sample were used to analyze the expression of CD3+ T cell CD28+,CD152+,CD14++ on mononuelear cell CD86+,and HLA - DR + by FACS expression.The relationship between CD28+,CTLA4,CD86+ and the HLA-DR +,and the relationship between APACHE Ⅱ Grading,CD28+,CD152+,CD86+ and HLA-DR + were analyzed.Results Compared with the control group,the expression of CD3 + T cell,CD86+ and HLA - DR + were remarkably reduced while the expression of CD28+ and CD152+ were markedly increased in patients with severe pneumonia who were hospitalized in 24 h(P＜0.05).However,T cells with positive CD8+ CD3+ and CD4+ CD3+ had no significant change between two groups(P＞0.05).For patients with severe pneumonia who survived,the APACHE Ⅱ scores were significantly reduced while the expression of CD28+,CD152+,CD86+,HLA-DR + and CD3+ + cells were significantly increased after 10 days from admission(P＜0.05).By contrast,T cells with positive CD8+ CD3+ and CD4+CD3+ had no significant change between two groups(P＞0.05).There were no relation between costimulators CD28+ and HLA - DR + (r=-0.12,P=0.54)and APACHE Ⅱ scores(r=-0.30,P=0.19) in control group.CD86+ and HLA - DR + showed positive correlation(r=0.65,P=0.00).CD86+ and APACHE Ⅱ scores had no correlation(r=-0.38,P=0.09).Conclusion The costimulators expressed abnormally in circumference blood of patients with severe pneumonia,CD86+ decreased,but CD28+,CD152+ increased.T cell of circumference blood was at the condition of "anergy".The increase of CD28+,CD86+,CD86+ and HLA - DR + during convalescence stages in patient with severe pneumonia showed that spocific immunity was advantageous for restoration in these patients.The relationship among CD86+,CTLA4 and HLA - DR + indicated that CD28+/CD152+:B7 play an role in the occurrence and development of severe pneumonia.