The present invention discloses the application of mesenchymal stem cells in the prophylaxis or treatment of stress response-induced weakened immunity. More particularly, the present invention provides the use of mesenchymal stem cells in preparing a pharmaceutical composition for the prophylaxis or treatment of stress response-induced lymphopenia and/or the use thereof in preparing a pharmaceutical composition for the prophylaxis or treatment of stress response-induced weakened immunity and/or the use thereof in preparing a pharmaceutical composition for the promotion of interleukin 4 (IL-4) expression and/or enhanced or strengthened activity. The present invention has found that mesenchymal stem cells can, by means of regulating IL-4 and down-stream related signaling pathways, protect against hormonal disorder-induced lymphopenia, thus having the function of activating immune response.
