cytokines, internalised?

[MJ Duggan]

Dear netters,

I know that several (all?) of the growth factors (e.g. NGF) are 
internalised after they bind to their receptors. But I would
like to know if this is true for the cytokines as well. I have tried
looking in several textbooks but can find no mention of such a
mechanism. Does this mean that the cytokines are not internalised,
that no one has looked (!), or that it happens but nobody thinks
that it is important? 

Any help, information, or pointers to the right place to look in the
literature would be most welcome.

Thanks

Mike Duggan