The alpha subunit of the IL5 receptor binds IL-5 with relatively low affinity. Binding of this dimer to the common beta subunit (Bc) confers high affinity binding. Recent models of receptor activation suggest a sequential activation that is initiated by the low-affinity interaction of ligand with the alpha chain to form a binary complex. This binary complex may bind preformed Bc dimers generating a 2:2:2 hexameric complex (Hansen et al. 2008).