Good tricky question,
Well Look at this in this way that there is an idea cell whose antenna can only receive in its beamwidth now for now a jammer is installed in its main lobe. now if you down tilt the antenna coverage would shrink, the receiver lobe too would move towards the the site and jammer would be out of the lobe so the site would not experiance interference.
Now in this example replace the jammer with a cell phone camped on to another site and the ideal antenna with a practical antenna which has greater loss outside its main lobe.
Its just a hypothetical answer hope it is ok .. still open for any questions