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• BSHY upregulates the numbers of lymphocytes in immunosuppressed mice. • BSHY increases proliferation and decreases apoptosis of lymphocytes. • BSHY enhances the proliferation of lymphocytes via IL-4/Stat6 signaling. The immune system plays a pivotal role in defending against infection and cancer immunosurveillance during the onset and procession of malignant disease. Cancer patients are frequently immunocompromised and subject to refractory infection and relapse of leukemia, due to the cytotoxic agents and immunosuppressive glucocorticoids in the chemotherapy regimens. Bu Shen Hui Yang Fang (BSHY), a traditional Chinese compound, was widely used in China to enhance the immune system of leukemia patients combined with chemotherapy and effectively lowered their risk of infection, with specific mechanism unknown yet. Thus, we investigated the effects of BSHY on the immune system using immunosuppressive mouse models. By analyzing the immune system of immunosuppressed BALB/C mice induced by hydrocortisone, we found an increase of CD4+ and CD8+ lymphocytes in the spleens of mice after BSHY treatment. Furthermore, we found the enhanced immune system in BSHY treated group was due to increased proliferation and decreased apoptosis of lymphocytes. Cytokine array analysis revealed that interleukin 4 (IL-4) was reduced in the plasma of immunosuppressed mice but returned to a normal level after BSHY treatment. Moreover, we found IL-4 was an adverse prognostic factor in acute myeloid leukemia patients and part of them could be elevated by BSHY. Mechanistically, we found BSHY enhances the proliferation of lymphocytes in a Stat6-dependent manner. In summary, our current study demonstrates that BSHY enhances the proliferation of lymphocytes in the immunosuppressed mice via upregulating IL-4 signaling. [ABSTRACT FROM AUTHOR]