IL-4 is produced mainly by a subpopulation of activated T cells (Th2) which are the biologically most active helper cells for B cells and which also secrete IL-5 and IL-6. Another subpopulation (Th1) also produces IL-4 albeit to a lesser extent. Non-T/Non-B cells of the lineage of mast cells also produce IL-4. The extracellular domain of the IL-4 receptor is related to the receptors for Epo, IL-6, and the beta chain of the IL-2 receptor. It has been given the name CD124. Two types of IL-4 receptor (IL-4R) exist: the type 1 receptor is a heterodimer consisting of CD132 and IL-4R-alpha. The type 2 receptor is a heterodimer consisting of IL-4R-alpha and IL-13R-alpha-1. The biological activities of IL-4 are species-specific; mouse IL-4 is inactive on human cells and human IL-4 is inactive on murine cells. IL-4 promotes the proliferation and differentiation of activated B cells, the expression of MHC class 2 antigens, and of low affinity IgE receptors in resting B-cells. IL-4 is probably an autocrine growth modulator for Hodgkins lymphomas.