Interleukin 13(IL-13)is a recently described novel cytokine produced by activated helper T cells. IL-13 protein is secreted mainly in unglycosylated form with molecular mass of about 10 ku.The mature ILs-13 of human and mouse consist of 112 and 111 amino acid residues respectively.IL-13 not only regulates the growth,surface antigen expression,and cytokine production of monocytes,but also regulates its proliferation,Ig isotype switching,and Ig synthesis of B cells.Furthermore,IL-13 synergizes with other hematopoietic growth factors in regulating proliferation and differentiation of primitive hematopoietic progenitor cells.