system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the body become ‘sensitised’ to certain chemicals whereby the chemical now becomes a serious allergy when it would not have been allergy in the past?

I was reading up on Black Henna Tattoos and it mentioned that this type usually has really strong doses of a certain chemical that can cause you to become allergic to dyes (I.e,. Hair dyes) in the future. What changes does the body go through that makes you allergic to the dye?
gpt
An allergic reaction is when your immune system overreacts to something foreign.

For things like bacteria or viruses, we can develop our immune system to fight them better the more we are exposed to them.  Every time your immune system encounters these threats, it builds more weapons to identify and fight then. This is how we develop resistance.

The problem with allergic/anaphylactic reactions is that we don’t want our immune system to be reacting to something,  but because our immune system learns and adapts, it gets *stronger* every time it is exposed to the foreign thing. 

So if you’re allergic to bee stings here’s what might happen. You get stung by a bee the first time and your immune system identifies the venom as a significant threat. The first time you get stung you may not experience much because your system is learning about it and developing weapons to fight it next time it comes around.  The second time you get stung, you could go into full anaphylaxis because your body is prepared for an “all hands on deck” response. Your body overreacts to the point of potentially killing you.  Legitimate responses to fight your bodies, invaders (swelling for instance) go completely overboard.