Dictionary.net

Soluble factor produced by activated T-lymphocytes that causes proliferation and differentiation of B-cells. Interleukin-4 induces the expression of class II major histocompatibility complex and Fc receptors on B-cells. It also acts on T-lymphocytes, mast cell lines, and several other hematopoietic lineage cells including granulocyte, megakaryocyte, and erythroid precursors, as well as macrophages.