Abstract: Ohjective To explore the value of peripheral blood monocyte subsets and their CD169 expression levels in the diagnosis ofcommon respiratory viral infectioms and the immume mechanism of the infection in children.Methods A total of 125 children with re-spiratory vinal infections ( virus infection group), 71 children with bacterial respiratory infections ( bacteria infection gmoup)，and 35healthy children ( healthy control group) admitted to the Third Affiliated Hospital of Zhengzhou University fromFebmuary 2022 to Octo-ber 2023 were incluled in this studly. The counts of peripheral blood monocytes and lymphocytes were detected by hematologic analyzer.The proportions of lymphocyte subsets , monocyte subsets , and the expression levels of CD169 were detected by a flow cytometry.Mono-cyte subsets were classified into three types : classical monocytes(CM) , intemedliate monocytes ( IM) ， and non-classical monocytes(NCM).ANOVA and Kruskal-wW allisHl tests were used to screen for differentially expressed indicators among the three groups , and thereceiver operator characeristic ( ROC) curve was used to evaluate their diagnostic performance.Paired t-lests were used to analyzre thedifferences in the imdicators between the first day and the seventh day of hospitaliation in the virus group.Spearman rank comelation a-nalysis was used to explore the relationmship between CD169 expression levels in monocyte subsets and the quantity of lymphocyte subsets.ResultsStatistically significant differences in the expression levels of CD169 in monoytes of CM,IM, and NCM types were ob-served in peripheral bloodl between the children's respiratory viral infection group anmd the healthy control group ( all P<0.01). In con-trast， no statistically significant differences in CD169 expression levels were found in monocytes of CM and IMtype between the respin-tory viral infection group anmd the bacterial infection group (all P>0.05). The differences in CD169 expression levels among the groupswere statistically significant in the NCM subset ( P=0.008) . The area under the ROC curve ( AUC) of CD169 exwression level in NCMtype monocytes for diagnosing respiratory viral infections was 0.897 (P<0.01)，with a cutoff point of ≥62.75%，sensitivity of68.60% , and specificity of 91.80%. The quantity and propotion of CM ,IM, and NCM type monocytes did not show statistically signif-icant differences between the respiratory viral infection group and the bactenial infection group ( all P>0.05).However, there were sig-nificant differences in the quantity and pmopotion of CM and IM type monocytes, as well as the proportion of NCM type monocytes , be-tween the virus inlection group anmd the healhy control group (all P<0.05). The proportions of CM , IM , and NCM type monocytes, thequantity of IM type monocytes , and the CD169 expression levels of moncyte sulsets showed statistically significant differences betweenthe first day and the seventh day after admission ( all P<0.05 ).There were no statistically significant diflerences in the quantity of CMand NCM type monocytes between the fist day and the seventh day after admission ( all P>0.05 ). The expression levels of CD169 imperipheral blood CM,IM，, and NCM type monocytes were negatively corelated with the mumber of CD8*T cells with corelation coefi.cients of -0.65,-0.63 , and -0.66，respectively ( all P<0.01). ConcusionThe expression level of CD169 in NCM type monocytescould be usedl as an auxiliary diagnostic marker for the children with respiratory viral infection.The changes of monocyte subsets duringrespiratory viral infection course in children may involve that more CM type monocytes differentiale into lM type monocytes , and someIM type monocytes do not diflerentiate into NCM type monocytes in proportion.IM anml NCM type monocytes may play important roles inthe early stage of respiratory viral infections in children.CD169 serves as a marker of monocyte activation.