Density dependent QCD effects in pp collisions - Unexpected revelations and valuable insights at LHC energies

Starting with a short introduction into the study of the Quark Gluon Plasma in heavy-ion collisions we motivate the interest in multiplicity dependent effects in proton-proton (pp) collisions. After reviewing basic measurements related to multiple partonic interactions we present some of the most intriguing results: collectivity and baryon enhancement in pp. The discussion of biases in the measurements of particle yields as function multiplicity brings us back to the study of heavy
ion collisions, where these effects also need attention for the correct interpretation of results.