Tenko Shimura's ‘allergies’ and the implications thereof
I think most people always agreed that Tenko's allergies were - in some form or another - emotion-based. Back in 235 we get the infamous "it only itches at home line" line.
It's also emphasized again later in the chapter when we see that the itching gets progressively worse after Kotaro yells at Tenko (meaning that it gets worse when Tenko is sad and/or in distress).
Back in the day, I saw a lot of people theorizing that the itch was a sign of Decay and that it was the early stages of the quirk manifesting within Tenko. But given what we know now, about AFO giving Tenko the quirk and especially when he gave Tenko the quirk, I don't think that this is the case.
We can pretty much pinpoint exactly when AFO gave Tenko the Decay quirk.
These two panels are from 235 and 419 - and I firmly believe that these panels take place only minutes apart from each other. This is also clearly the moment that AFO gave Decay over to Tenko, as shown by the small glow of his hand. Before this point Tenko Shimura was quirkless.
Tenko got his original quirk stolen from him when he was just a baby - meaning that during the time between being an infant and being 5-years-old, Tenko was officially quirkless. This makes a huge difference if we then look back at his allergies and why they appear.
This panel is from just before AFO takes Tenko's hand and gives him the Decay quirk. Look at his face. This boy already has irritated skin around his eyes (it looks like he's just been scratching at it too). With the knowledge that Tenko at this point in time still is quirkless, I think it's fair to say that the allergies are not an early symptom of Decay. It only itches at home. Because the itching is purely psychosomatic. Because it's a stress-response/anxiety-response to the environment this young boy is forced to come home to every single day.
This boy is 5. He's 5. It's not normal for a 5-year-old to think thoughts anywhere near "does my father hate me?"
Kotaro has already put a fear in this little boy, which doesn't stop or go away as soon as he's punishment does. The trauma in this boy is already so big that he thinks about these things in the moments when his dad isn't even actively scolding or punishing him.
It doesn't take a destructive quirk to cause a physical response like itching. All it takes is being constantly afraid of when you're going to be yelled at next, when you're going to be punished next, all by the hands of the parental figure you aren't sure even loves you at all.
It only itches at home. Especially when the four walls of your house are a prison.
