Clinical Information

Interleukin-5 (IL-5), a homodimer composed of two 20-kDa subunits, is expressed primarily by CD4+ Th2 (helper T cells, subset 2) cells and, to a lesser extent, by activated mast cells.(1,2) IL-5 acts on mature eosinophils, leading to proliferation, activation, differentiation, and survival.

IL-5 plays a critical role in the host immune response to helminthic infections and has been implicated in the pathology of certain allergic diseases, asthma, and vasculitis.(1-3) In these diseases, it is associated with significant increase in levels of eosinophils, a condition referred to as hypereosinophilia. IL-5 is known to activate eosinophils, which interact through their Fc receptors to bind helminths that have been opsonized by IgG or IgA specific antibodies.(1) Of the other diseases associated with IL-5 and hypereosinophilia, those of the skin and airways (urticaria, asthma, allergic bronchopulmonary aspergillosis, and eosinophilic granulomatosis with polyangiitis [EGPA]) have received considerable attention recently due to availability of therapies that target the IL-5 pathways.(4) In EPGA, increased levels of IL-5 have been seen in a subset of patients with implications for disease management.(4,5) The IL-5 pathway-directed therapies have been approved for use in patients with severe eosinophilic asthma. With availability of these therapies, it is likely that IL-5 cytokine testing may be used to identify patients at-risk for disease.