Why do hemopoietic growth factor receptors interact with each other?

Hemopoietic growth factors (colony-stimulating factors, CSFs) interact with distinct cellular receptors that recognize only their cognate ligand. Yet, like other growth factor/receptor systems, the binding of one type of CSF to its receptor can 'down-modulate' the availability of a different tyoe of CSF receptor. In this article Nicos Nicola discusses the distinctive pattern of] CSF-receptor modulations and suggests a novel interpretation of such modulations as a means of coupling receptor-derived signals which is consistent with the known biology of the system.