Mesenchymal stem cells (MSCs) are the adult stem cells originating from the mesenchymal and connective tissue of bone marrow, adipose tissue, placenta, umbilical cord, cord blood, peripheral blood, liver, etc. These cells show immunomodulation, self-renewal, and multi-directional differentiation potential. In particular, MSCs have recently emerged as promising candidates for cell-based immunotherapy because they can modulate the immune response in various ways. A series of studies have indicated that secretion of dissoluble cytokines and direct contact with MSCs can block the development and functioning of antigen-presenting cells, inhibit the differentiation of B cells, and suppress the immune response of T cells and natural killer cells. The immunosuppressive effect of infused MSCs has been successfully employed in the treatment of acute severe graft-versus-host disease (GVHD). Thus, MSC may reduce inflammatory responses and promote tissue recovery in human diseases.