Homodimeric cytokine expressed predominantly by T-lymphocytes and NK cells that plays an important role in the survival, differentiation, and chemotaxis of eosinophils (PubMed:10444455,PubMed:1873482). Acts also on activated and resting B-cells to induce immunoglobulin production, growth, and differentiation (PubMed:3128631). Mechanistically, exerts its biological effects through a receptor composed of IL5RA subunit and the cytokine receptor common subunit beta/CSF2RB. Binding to the receptor leads to activation of various kinases including LYN, SYK and JAK2 and thereby propagates signals through the RAS-MAPK and JAK-STAT5 pathways respectively (By similarity). Homodimer; disulfide-linked. Interacts with IL5RA (PubMed:1873482). Interacts with CSF2RB. In IL-5 deletion mice, the intestinal eosinophilia that developed in WT mice during T. spiralis infection was completely abolished, revealing the importance of IL5 for the increased production and recruitment of eosinophils to the gut during infection. Belongs to the IL-5 family.