Interleukin-5 (IL-5) is a secreted glycoprotein that belongs to the alpha -helical group of cytokines . Unlike other family members, it is present as a covalently linked antiparallel dimer. The cDNA for human IL-5 encodes a signal peptide and a 115 amino acid mature protein. Mature human IL-5 shares 70%, 70%, 62%, 71%, 70% and 66%, aa sequence identity with mouse, rat, canine, equine, feline and porcine IL-5, respectively and shows crossreactivity with mouse IL-5. IL-5 is primarily produced by CD4+ Th2 cells, but also by activated eosinophils, mast cells, EBV-transformed B cells, Reed-Sternberg cells in Hodgkin’s disease, and IL2stimulated invariant natural killer T cells (iNKT) . IL-5 increases production and mobilization of eosinophils and CD34+ progenitors from the bone marrow and causes maturation of eosinophil precursors outside the bone marrow. The receptor for human IL-5, mainly expressed by eosinophils, but also found on basophils and mast cells, consists of a unique ligand-binding subunit (IL-5 R alpha ) and a shared signaltransducing subunit, beta. IL-5 R alpha first binds IL-5 at low affinity, then associates with preformed beta c dimers, forming a high-affinity receptor.