What exactly is it?
As Professor Lupin told Harry in Prisoner of Azkaban, the Patronus is ‘a kind of Anti-Dementor – a guardian which acts as a shield between you and the Dementor.’ It’s also ‘a kind of positive force, a projection of the very things that the Dementor feeds upon – hope, happiness, the desire to survive – but it cannot feel despair, as real humans can, so the Dementors can’t hurt it.’

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