Description: Human IL-10 is a ~18 kDa factor also known as cytokine synthesis inhibitory factor (CSIF). Th2 T cells, B1 B cells and keratinocytes are the major cell subsets that produce IL-10. IL-10 inhibits synthesis of Th1 cytokines and proliferation of T cells and acts as a costimulatory signal for mast cells, developing thymocytes and Th2 response.