system
Given the following post, respond as if you're commenting on Reddit:
human
How is it possible to treat allergies by repeatedly presenting the antigene to desentize but when you vaccinate (and booster) an antigene presentation trains the immune system to fight said antigene?
gpt
A classical vaccine typically contains two things: an antigen and an adjuvant. The antigen is what you're training the immune system to recognize, and the adjuvant is a more generic substance that lets the immune system know there's something dangerous going on.

If you only give an antigen, that danger sign is absent, and the immune system instead "learns" that this antigen is not dangerous.

PS: Newer vaccine technologies sometimes don't include the actual antigen, but instead a recipe for making it (either mRNA or DNA). mRNA vaccines also don't always need the adjuvant, because encapsulated RNA itself can work as a "danger signal."