Interleukin 5 (IL-5) has also been described as B-cell growth factor II due to its induction of proliferation and immunoglobulin secretion by activated B cells. It belongs to the group of cytokines that include IL-3, IL-5, and GM-CSF. Many other activities have also been attributed to this cytokine, including the development and differentiation of eosinophils from bone marrow stem cells, the induction of IL-2R membrane receptor expression in activated B cells, and increased IgA secretion by LPS-stimulated B cells. IL-5 is produced by the Th2 subpopulation of helper T-cells, while smaller amounts are secreted by activated eosinophils and mast cells. The range of biological activities of IL-5 in B cells and other cell types suggests that IL-5 plays an important functional role in Th2 cell activity. IL-5 is a highly glycosylated glycoprotein with a molecular weight of 45 to 60 kDa.
In humans, IL-5 primarily acts on eosinophils and promotes their differentiation, maturation, activation, migration, and survival. Although eosinophilic lung infiltration is a feature of allergic asthma that may be driven by IL-5, the therapeutic administration of anti-IL-5 antibodies did not appear to alter asthma symptoms in clinical trials. Administration of anti-IL-5 antibodies can, however, reduce circulating eosinophils by 50% and has shown good efficacy in treating hypereosinophilic syndrome. IL-5 has also been reported to be a differentiating factor for basophils and may promote their secretion of histamine and leukotrienes.
