A Patronus is an advanced form of magic, a powerful defensive charm that only the most advanced wizards can cast, Harry Potter of course, is an exception and capable of casting his first at the age of 13.They protect the wizard or witch against the Dementors by projecting everything Dementors feed upon, hope, happiness and the desire to survive, but unlike humans, they are quite incapable of feeling despair, thereby neutralising the effects of Dementor attacks.
