system
Given the following post, respond as if you're commenting on Reddit:
human
Why do mosquito bites itch?
gpt
When mosquito bites you, the inject a "foreign object".  It's foreign because it's not made by your body.  So when the body detects the foreign object, it's response is to destroy it.  But to destroy it, you need special "fighter" cells, which have the "guns and knives" to destroy the object.  When you scratch your itch, it warms up that part of your body, which signals the special "fighter" cells to come down there and start attacking the foreign object.

So the more you scratch, the more warmer that part of your body gets, and more fighter cells come.  It's good if a lot of fighter cells come so they can quickly destroy the object.  

But imagine if it's a war and if you have too many soldiers and tanks, you will have a done lot of damage to the battlefield.   Same thing in your body, if a lot of "fighter" cells come, it gets really bloody in that area because your blood brings the cells there.