Recombinant human interleukin-5 (rhIL-5) is a selective activator of human eosinophil function. Eosinophilia is a clinical condition characterized by an increase in eosinophils, often seen in allergic diseases, parasitic infections, and hypereosinophilic syndromes. The study shows that rhIL-5, a recombinant human cytokine, stimulates the proliferation, differentiation, and function of eosinophils. It is the first hemopoietic factor shown to be involved in the selective eosinophilia and activation of eosinophils in disease. rhIL-5 induces morphological changes in eosinophils, such as membrane ruffling, elongation, and granule concentration, which are typical of activation. It also stimulates eosinophils to kill antibody-coated tumor cells and opsonized yeast, while neutrophils are not affected. rhIL-5 also enhances the production of superoxide anion by eosinophils, similar to rhGM-CSF. However, rhIL-5 does not stimulate superoxide production by neutrophils, unlike rhGM-CSF. The study suggests that IL-5 is likely responsible for the increased eosinophil numbers and activation seen in various diseases. IL-5 may have clinical implications, such as aiding in the control of parasitic infections and contributing to tissue pathology in hypereosinophilic syndromes. It may also be produced by certain tumors and is associated with paraneoplastic syndromes. IL-5 is the third human colony-stimulating factor, after GM-CSF and IL-3, capable of stimulating eosinophil proliferation and function. Its ability to activate eosinophils is not obviously due to similarities in primary structure but may be related to homology in the carboxy-terminal region.Recombinant human interleukin-5 (rhIL-5) is a selective activator of human eosinophil function. Eosinophilia is a clinical condition characterized by an increase in eosinophils, often seen in allergic diseases, parasitic infections, and hypereosinophilic syndromes. The study shows that rhIL-5, a recombinant human cytokine, stimulates the proliferation, differentiation, and function of eosinophils. It is the first hemopoietic factor shown to be involved in the selective eosinophilia and activation of eosinophils in disease. rhIL-5 induces morphological changes in eosinophils, such as membrane ruffling, elongation, and granule concentration, which are typical of activation. It also stimulates eosinophils to kill antibody-coated tumor cells and opsonized yeast, while neutrophils are not affected. rhIL-5 also enhances the production of superoxide anion by eosinophils, similar to rhGM-CSF. However, rhIL-5 does not stimulate superoxide production by neutrophils, unlike rhGM-CSF. The study suggests that IL-5 is likely responsible for the increased eosinophil numbers and activation seen in various diseases. IL-5 may have clinical implications, such as aiding in the control of parasitic infections and contributing to tissue pathology in hypereosinophilic syndromes. It may also be produced by certain tumors and is associated with paraneoplastic syndromes. IL-5 is the third human colony-stimulating factor, after GM-CSF and IL-3, capable of stimulating eosinophil proliferation and function. Its ability to activate eosinophils is not obviously due to similarities in primary structure but may be related to homology in the carboxy-terminal region.