Increased morbidity and mortality has been recorded from diseases such as HIV/AIDS, tuberculosis, hypersensitivity reactions, autoimmune disorders, and graft versus tissue rejections. Therefore, immunomodulation can be beneficial depending on the desired immune status. The aim of this study was to determine the effect of aqueous extract of leaves of Ocimum gratissimum on immune response in albino Wistar rats. Albino Wistar rats were administered the following extracts/drugs by oral administration for 21 days: Group 1 (40 mg/kg extract), Group 2 (80 mg/kg extract), Group 3 (60 mg/kg extract), Group 4 (Levamisole 0.5 mg/kg) positive control, Group 5 (no drugs, no extract) negative control. After 21 days, blood samples were collected for the estimation of plasma IgG level and absolute WBC count. There were statistically significant reductions in plasma IgG levels in the test groups compared to controls (p value 0.052). Multiple comparisons (Dunn’s) of the group values further revealed statistically significant p values of 0.022 (group 2 vs group 1), 0.023 (group 2 vs group 4), and 0.011 (group 5 vs group 2). There were also statistically significant reductions in total WBC in the test groups compared to controls (p value 0.043). Multiple comparisons (Dunn’s) of the group values further revealed statistically significant p values of 0.013 (group 2 vs group 5), and 0.010 (group 3 vs group 5). At the doses and concentrations used in this study, the aqueous leaf extract of O. gratissimum produced dose dependent inhibition of plasma IgG concentration and total WBC count in albino Wistar rats