The ionization state of the jet material varies considerably within the jet due to the presence of dust which can scatter the harder radiation.  Nonetheless, many heavy ions are also present in these jets.   The jets in AGNs are physically different than those in X-ray binaries.  X-ray binaries are basically accretion flows with some collimation, where as AGN jets are much more energetic and more highly collimated and are not accretion flows, but rather ejected collimatted  and highly ionized matter.

I think pure leptonic jets are not  possible in nature but they may have laptonically dominated flow over other heavy ions or molecules because these jets basically originated from the disc around the central objects.  Pure electron-positron flow is mostly non-relativistic and having very low temperature(around 10^7K), so at very low temperature, e^--e^+ pairs are unstable..for reference you can see Kumar, R, etal. 2013, MNRAS.

Thanks Bothun and Rajiv for your answers.