Plot Allergy

Allergies are no fun but they can be a useful device in fiction.

Want Alice to be able to go somewhere where Bob can't but don't want to introduce fantasy elements? Have her start coughing whenever she gets near it. Want a tough character to be really afraid of just one thing? Have him be allergic to it.

This trope manifests in several different forms, among them:
 * The movement restriction allergy: "I can't go there! I'm allergic to X!"
 * The relationship hitch allergy: "My new girlfriend has a cat/sent me flowers, guess what I'm allergic to"
 * The detection allergy: "We need to find / be warned about a certain creature, and happen to be allergic to it!"
 * The assassin allergy: "He was killed with he previously unknown allergy to a X!"
 * The manipulation allergy: "I have your allergen, you will obey me!"

In real life allergies can cause a number of problems from sneezing to death by throat closure, but fictional examples will usually just include sneezing, streaming eyes and occasionally hives. Also covered would be 'intolerances', which generally cause stomach troubles e.g lactose intolerance.

Please note: Danger is not an actual allergy. Neither is Love

Can results in a Sneeze of Doom.

Comic Books

 * In Runaways, Gert doesn't want to fight a Nazi made of bees because she is allergic to them.

Films - Animated

 * Cloudy with a Chance of Meatballs: Sam's peanut allergy prevents her from going down to the food-making machine with Flint through a pit surrounded by shards of peanut brittle. She lowers him down using a licorice rope,

Video Games

 * Both Adventures of Sonic the Hedgehog and Sonic the Hedgehog use this with Pseudo Sonic. Sonic ends up being launched into a field of flowers which causes him to swell up like a balloon. Tails saves the day in both adaptations, but the comic has Bunnie Rabbot coming to Sonic's rescue before Robotnik can get rid of him.

Films - Animated

 * In The Cat Returns, the cats fill Haru's front garden with cattails. In the English dub she later tells the messenger that she's allergic to them. (Actually a slight change from the Japanese version, but given that Haru woke up sneezing the morning after they were planted it's hardly contradictory.).
 * Though this definitely doesn't apply to the manga version of the same scene, as the cattails were planted elsewhere. It may be that Haru was just sneezing from the cold, as she'd been sleeping largely uncovered.

Live Action TV

 * In an early episode of The Brady Bunch Jan develops an allery to Tiger, the pet dog. They plan to give the dog away. But then Jan discovers that she's not allergic to Tiger, she's just allergic to Tiger's new flea powder. (Ironically this was the last episode to feature Tiger as a major character - it's not that somebody was allergic to him, it's that in Real Life he got hit by a car. One other episode talks about Tiger a lot but he's not actually on screen much - he's missing.)

Western Animation

 * One Garfield and Friends episode, his owner Jon finally finds a girl...But it turns out that she is allergic to cats at the very end, and the status quo is restored.
 * Wallace and Gromit: Wallace's first potential love interest (Wendolene) is allergic to cheese, dooming the relationship.

Web Comics

 * Questionable Content:
 * A bump on the road in Faye's relationship here with Angus happens when she give him a coffee with milk and he ends up spending some time in the toilet in gastrointestinal distress.
 * Shortly after Marten starts dating Dora, he surprises her with some roses. The inevitable result is anaphylactic shock, but contrary to the more common results of this trope, Dora actually feels sorry for Marten since he's had an attempt at romance blow up in his face...
 * In PvP, at one point, Skull the Troll briefly dates a girl who works in a nearby cafeteria. A while later, a mutual friend reminds him that he's been neglecting her, and he rushes off to see her with some flowers. Only, it turns out that she's allergic to them, and he's forgotten it. Being somewhat Genre Savvy, he knew that might happen, and ALSO brought chocolates. Which, as it turns out, she's also allergic to.
 * In Las Lindas, during the 'party' arc, Miles' "The Reason You Suck" Speech for his girlfriend/ex-girlfriend Taffy gets countered by one of these. When he complains that he'd never know if "one more bouquet of flowers" could get her to finally put out, she simply says "I'm allergic to flower pollen. You'd know that if you'd ever gotten me any." Ouch.
 * Also, atypically enough, her flower allergy did come up earlier, in a non-romantic example of this trope - Digit tried to give her a bunch of flowers to say she was sorry about a previous mishap, only for the flower allergy to ruin her attempt.

Films - Animated

 * The Secret of NIMH: Jeremy's cat allergy, warning them that 'dragon' the farmers cat is near and distracting him when he does arrive.

Literature

 * In the Goosebumps book I Am Your Evil Twin, the main character uses his peanut allergy to distinguish himself from his evil clone—since they aren't sure how allergies work, he supposes that the clone might not have it. (After the climactic test, though, it turns out that the clone swapped out the test food for one that he wasn't allergic to, and then made himself throw up so everyone would think he was the real thing.)
 * Taken Up to Eleven by Ruby "Pufferfish" Peet in  N.E.R.D.S. This child's allergies are so severe they go off when exposed to things such as danger,threats, dishonesty, betrayal. Making her a human danger detector.

Live Action TV

 * NCIS: McGee is allergic to a cologne of a suspect, later a body turns up that causes McGee to sneeze.

Western Animation

 * Teen Titans: Starfire's allergy to a metal used in a certain bomb (and lots of other stuff)

Video Games

 * Snatcher uses a Detection Allergy- the eponymous robots release a masking agent called SNOW-9 which is similar to pollen, causing people (including the protagonist) to sneeze when they're in the area.

Films - Live Action

 * Black Widow. Black Widow Catharine Petersen marries men and murders them by giving them a substance to which they have a lethal allergy.

Literature

 * In the last book of the Empire of the Ants trilogy, the protagonist's uncle dies near the secret base because he turns out allergic to their sedative.

Live Action TV

 * The the Doctor Who TV Movie, the Doctor was killed by some well-meaning Doctor who gave him human medicine that he was allergic to. This being the Doctor, he got better.

Western Animation

 * The BOTS Master Ziv Zulander and his sister almost fell victims to this trope.

Live Action TV

 * Just about all the husbands on Desperate Housewives have suffered from some convenient allergy which their wives use to manipulate in some way, which is then never mentioned again. Especially obvious in the case of Carlos, who was allergic to eggs for one episode, despite this never inhibiting anything he's eaten in previous and later episodes. You could Fan Wank that all his food is made with egg substitutes and it's just never mentioned, but what all those times he's shown eating scrambled eggs for breakfast?

Western Animation

 * In The Simpsons Bart finds out about Principal Skinner allergy (Nuts) and uses it to manipulate him, late it turns out Bart has an similar allergy (Shrimp) and violence ensues.

Anime & Manga

 * Anime gives us an example of allergy as code phrase: in Weiss Kreuz Verbrechen & Strafe, one of the operatives signals her team that something is amiss by referring to a flower allergy she doesn't actually have. (They know she's faking because the other operatives are all florists.)
 * An episode of the first Dirty Pair TV series had as the antagonist a man allergic to women. Mostly played for laughs.

Film

 * In The Man Who Knew Too Little, Wallace complains several times about Boris "The Butcher" wearing Old Spice; his violent sniffling and sneezing actually helps him to escape Boris' custody at one point. At the very end, we learn that it wasn't Old Spice causing Wallace's reactions—it was an otherwise-undetectable Amazonian arrow-frog toxin.
 * Addicted To Love: Meg hides crushed strawberries under her ex's pillow, causing him to break out in an embarrassing skin rash and sabotaging his modeling career.

Live Action TV

 * In one episode of Imagination Movers, the Movers get their Idea Emergency when they need to figure out why Dave keeps sneezing. Turns out, he was allergic...to wool (Scott was wearing wool socks).
 * On The Big Bang Theory, Howard needs to keep Leonard away while the others plan a surprise birthday party for Leonard, so he uses his peanut allergy as an excuse to take Leonard to the hospital. When things get desperate, Howard has to actually trigger his allergy to keep Leonard from going back.

Webcomics

 * Double subverted in Homestuck. John is mentioned early on to be deathly allergic to peanuts, but it doesn't come up for him beyond a few narration asides. Later on, Jake's dreamself, who is, is discovered dead. Cause of death? The Courtyard Droll force-feeding him peanuts in his sleep.
 * Of course, being Courtyard Droll, he actually just stuffed so many unshelled peanuts in his mouth that he choked.

Western Animation

 * Butch Cassidy and the Sundance Kids. The artificially intelligent computer who sent the team on missions was allergic to dogs. Guess which animal was the Team Pet.
 * Played with in the Regular Show episode "Eggscellent". Rigby enters a contest where he must eat a gigantic twelve-egg omelet in order to win a hat. Problem is, he's allergic to eggs. This does not stop him from trying anyway (he really wants that hat!), and a Gilligan Cut later he's being rushed to the hospital.
 * Franklin had one in which Franklin kept sneezing. The characters tested a number of possible causes and finally concluded that he was allergic to his best friend, Bear, as he only sneezed when he was around Bear or handled things that Bear gave to him. The two stayed apart for a while, but Franklin eventually decided he didn't mind sneezing if he could play with his best friend. He then found that he didn't sneeze around Bear when Bear was covered in mud. Then, when he went to Bear's house, he got a bath and it was discovered that it was actually the new bubble bath that Bear was using that was causing the sneezing.