Active galactic nuclei with a relativistic jet pointing to the Earth, also called blazars, are natural accelerators of particles, as witnessed by the strong non-thermal emission. This makes them good candidate sources for extragalactic cosmic rays and neutrinos at high-energy.
The recent detection of IceCube-170922A, a $\sim 300$ TeV neutrino potentially correlated with the flaring blazar TXS 0506+056, directs attention toward this kind of objects as neutrino emitters. This coincidence event shed light on the structure and dynamics of the sources confirming the presence of cosmic rays inside the jet. In particular it offers a unique opportunity to explore the interplay between energetic photons, neutrinos and cosmic rays in the jet.