OBJECTIVE: To study the cause of low immunologic function and insufficiency of immunoglubulin synthesis in neonates by detecting CD21 expression in B lymphocytes in different age group children.MethodsThis study consisted of three age group children: 2-26 days (n=18), 6 months-2 years (n=12) and 3-12 years (n=17). CD21 expression in B lymphocytes was detected with flow cytometry. Serum levels of immunoglobulins were measured by immunoturbidimetry. RESULTS: The percentage and the number of B lymphocytes expressing CD21 in the neonate group were significantly lower than in the other two age groups. The neonate group also showed lower mean fluorescence intension (MFI) of CD21. The percentage and the number of B lymphocytes expressing CD21 as well as the MFI of CD21 increased significantly with the age. The serum levels of IgA and IgM in the neonate group were noticeably lower than those in the other two age groups. The serum levels of IgA and IgM also increased significantly with the age. CONCLUSIONS: Low CD21 expression in B lymphocytes may be related to low function of humoral immunity in neonates.［Chin J Contemp Pediatr, 2007, 9 (6):546-548］