How did Luffy become immune to poison in One Piece?

Long before several characters in Demon Slayer postured about their immunity to poison, One Piece went there first. But Luffy’s immunity to poison wasn’t didn’t come about by simply mentioning something which happened off-screen ages ago. No, Luffy’s journey to poison near-immunity happens very much on-screen (or on-page, if you’re reading the manga). And he nearly dies because of it.

The on-screen event in question happens during the Impel Down Arc, which is the penultimate arc before the time skip. With the help of the Pirate Empress, Boa Hancock, Luffy has infiltrated Impel Down, the Navy’s premier and seemingly impenetrable prison complex. Luffy’s brother, Ace, is being held on Level 6 ahead of his scheduled beheading at Marineford. Level 6 happens to be the bottom-most layer furthest from the sea-bound prison’s entrance. But because he is the fiercely loyal and reckless lad we all love, Luffy is determined to save Ace.

Once Luffy descends to Level 4, however, a fierce foe stands in his way: Magellan, the warden of Impel Down. Magellan has even the Venom-Venom Fruit, which allows him to produce and control different kinds of poison. It also allows him to eat poisonous foods and merely have terrible, hours-long diarrhea. But that’s not relevant to the fight.

In combat, Magellan’s poison is so deadly that, once it touches you, you’re done for. Unfortunately, at this juncture in time, Luffy’s combat style depends entirely on being able to make contact with his foe. While Luffy is initially content to sacrifice his arm, Magellan succeeds in overwhelming him and drenching his entire body in poison. Magellan then leaves an unconscious Luffy to die.

Of course, this is a huge mistake. Luffy’s pal Bon Clay drags him all the way to Level 5, where they are eventually found by Inazuma. Inazuma leads the duo to Emporio Ivankov, who wields the Hormone-Hormone Fruit. Ivankov injects Luffy with their Healing Hormone. However, Luffy’s condition is so dire, the Healing Hormone is not guaranteed to work. Fortunately, Luffy’s ridiculously strong will to live gives him enough edge to pull through.

There’s an upside to this harrowing and incredibly painful experience. After exposure to such a high dose of poison and hormones to counteract it, Luffy’s body can now produce antibodies against deadly poison. Our dear Doctor Chopper has verified this. However, the presence of these antibodies does not make Luffy 100% immune to poison. It’s more like he has incredibly high natural resistance.

To understand this, think of antibodies your immune system might develop after getting a virus – or from getting a vaccine. You’re not guaranteed full protection from getting the virus, but the antibodies make it less likely and help protect you from severe disease. So it is with Luffy. Not that he could get a vaccine against poison. There is a vaccine against certain viruses, though. You should get those vaccines. Huh? What?

Anyway, that’s the tale of Luffy’s quasi-immunity to poison. Reiju’s poison could still totally kill him. But kill him slower. Which is helpful.

(Image credit: Toei Animation)