Dear Monara, I'd prefer the polyclonal secondary, as long it does not give undesired background signals, because usually the signal to noise ratio with (cross-absorbed, of course) secondaries tends to be much better as you'll get more secondary molecules bound to your primary AB.

I agree with Wolfgang, the polyclonal secondary antibodies will enhance the signal since they can bind to various epitops on the primary antibody while the monoclonal has only one epitop where to bind...

Both answers are right.

Good luck!

The monoclonal can be better if you are concerned about cross-reactivity 

ok, thanks a lot! however, if I have already a mAb, the sencodary has to be monoclonal as well? Or it does not matter if I use mono or poly?

Thanks in advance, Monara :-)

If mAb works for you, it's fine. Polyclonal secondary will be better, just make sure it was not cross reactive to sample tissue.

I agree with all comments

Your secondary antibody does not have to be the same type as your primary antibody, monoclonal primary with polyclonal secondary is OK, as would be polyclonal primary with monoclonal secondary. It is usual for the secondary antibody to be from a different species to the primary antibody, so trying to match subclasses is not necessary. As other respondents have indicated the properties you are looking for are specificity to your primary antibody (to reduce background staining and cross reactivity), and high binding to your primary antibody (to increase signal). A polyclonal secondary is likely to give better amplification of the signal as it can bind to many different epitopes on the primary antibody.

Thanks everyone to take time answering me! :-)

Great Chris Selkirk, thanks for your answer!

Depending on the type of experiment, the choice of using a monoclonal or polyclonal secondary is most often trivial: Make sure you omit the primary in a parallel experiment so you can appreciate any potential signal coming from the secondary. An affinity purified polyclonal with minimal cross-reactivity to serum proteins could be superior to a monoclonal secondary for the reasons others already explained.

Very helpful and informative inputs