Lily's sacrifice leaves protective mark on Harry
When Voldemort attacked the Potters, Lily Potter died shielding her infant son, and her voluntary self-sacrifice left a protection in Harry's skin that makes Voldemort's curse rebound and later burns Quirrell's hands when he touches Harry. Dumbledore explains that love this deep leaves its own mark, one that someone full of hatred and greed cannot bear to touch. The book's central plot thus rests on an act of love working as a literal defensive force.
