Interleukin 5 (IL5)

IL-5 is secreted by a restricted number of mesenchymal cell types. In its native state, mature IL-5 is synthesized as a 115 aa, highly glycosylated 22 kDa monomer that forms a 40-50 kDa disulfide-linked homodimer. 

Although the content of  carbohydrate  is  high, carbohydrate is not needed for bioactivity.  Monomeric IL-5 has no activity and requires a homodimer for function, which is in contrast to the receptor-related IL-3 and GM-CSF cytokines that exist only as monomers.  Just as one IL-3 and GM-CSF monomer binds to one receptor, one IL-5 homodimer is able to engage only one IL-5 receptor.  It has been suggested that IL-5 (as a dimmer) undergoes a general conformational change after binding to one receptor molecule and this change precludes binding to a second receptor.