Rodney canNOT be "mortally" allergic to citrus. If he were, he'd be *dead* - do you know how many places use citrus oil as a cleaner? And t his is assuming he has another source of vitamin C because scurvy kills. It's also a very common ingredient in so many processed foods that he'd basically starve to death.
What I *do* buy is that he has a *mild* allergy to citrus oil - enough that he gets hives when he goes near too high a concentration. It's unpleasant (and, you know. Benadryl might well counteract it. Or Zyrtec. I have contact dermatitis. Zyrtec keeps me sane and hive free.) and, yeah, prolonged exposure will, in fact, make the allergy worse, and he's quite right to want to avoid it when possible. But touching a lemon? Will not kill him.
And we know he's a hypochondriac. His "hypoglycemia" is extremely atypical, for example. This, again, doesn't mean he doesn't have hay fever or isn't allergic to bee stings. He might well have a hyperactive immune system, which really isn't fun (see above re:contact dermatitis. Also hay fever that makes me wheeze and an inability to stay for long in a house with cats. All coming upon me at the same point, and all reducing and leveling off, thank God.) However, he's just the sort to exaggerate his symptoms from "annoying" to "deadly". And John is just the sort to both call him on it (Yes, I'm allergic. I can't breathe. Okay. I can breath, but it's not easy. Okay, but I get *covered* in hives. Well, not so much *covered*, but the place of contact can itch for an hour.") and tease him about it afterwards.
Which, honestly, I can't see him doing if it were actually serious - then it stops being a joke or funny. Also, he saw the whole lemon thing and smiled, so he wasn't exactly feeling betrayed.
I think the lemon thing was more a message from John to Rodney. "I believe you can do this. These guys don't know you the way I do, but I believe in you."
And, here's the thing. Over the past couple seasons, he's been *less* hypochondriacal as he's gotten more validation from everyone.
And, yeah. It was funny.
And, no. I really don't think John carries a lemon with him at all times. He doesn't have to. Rodney will save the universe because John believes he can.