Eosinophils are white blood cells that respond to parasitic infections. The growth and differentiation of eosinophils is selectively regulated by the cytokine IL-5 that exerts its influence through a receptor that is formed by two peptides, referred to as the IL-5 α and β common receptors. The details of how the presence of IL-5 is communicated to the interior of the eosinophil is still the subject of much research. It is thought that IL-5 binds the α chain of the receptor and that this complex binds to the β -chain that is bound to the cell surface. The βcsubunit is the main signaling component and also responds to the cytokines GM-CSF and IL3. The IL5 receptor system belongs to the class I family of cytokine receptors. Sequence analysis indicates that the βc subunit consists of 4 fibronectin (type II) domains that appear to pack into 2 cytokine homology modules. It has been assumed that the structures of these modules are similar to those of other Type 1 receptors - the recent structure determination of the βc subunit sheds new light on this idea.