system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do allergy shots work over time but not the allergen itself?

I understand how allergy shots work. What I don't understand is why they work by exposing you to the substance you are allergic to, yet just being exposed to your allergen over time doesn't build the same tolerance.
gpt
Imagine if you had parents (the immune system) who freaked out (allergic reaction), if you stayed up past your bedtime  which is 4pm (crazy low threshold)! They thought you'd actually turn into a monster if you were out of bed after 4pm, no one knows why, maybe they saw you cranky once and put the kibosh on the whole thing. Anyway. You have to convince them that you should have no set bedtime at all and they shouldn't freak out (no allergic response).

Option 1 - Stay up till 8, maybe 9 or 10, and see what happens. Or maybe midnight just to show them? (encounter the allergen) They'll flip out. They'll get mad. This definitely won't work. They'll get more vigilant and more strict and very upset. 

Option 2 - Stay up till 4:00:02 on "accident". Then 4:00:05, then 4:00:30, then 4:01. Over some months, keep overshooting by a tiiiiny and almost unnoticeable amount until 4:30pm, 5pm, 7pm... etc.

As long as they're not getting really upset, you can fudge it just a little and then they're comfortable with that new time. Eventually they will probably stop being so jumpy (less or no allergic reaction).