Abstract : The purpose of this study is to measure the amount of lymphocyte B cells and the level of Immunoglobulin G (IgG) as an indicator of infection susceptibility in DS patients. This research used case-control design. Samples were 20 DS patients, and 25 control non-DS, from 4 to 16 years old. DS patients and non-DS were identified by physical examination and chromosome examination. Venous blood was taken from each subject for chromosome examination, counting the number of lymphocyte B cells CD19+ and measuring IgG levels. Data were analyzed by Mann-Whitney test. This study found that the average number of lymphocyte B cells CD19+ in patients with DS was 305.05 ± 162.52, which was lower than the control group (750 ± 501.74) with p=0.00. The mean IgG levels in DS patients (1421.20 ± 363.37) were found to be higher than the control group (1291.80 ± 91.69), p>0.05. This study showed that the number of lymphocyte B cells CD19+ was lower in DS patients than controls. It may be due to the continuous activation of the immune system that will lead to apoptosis resulting in a decrease in the number of lymphocyte B cells CD19+.