system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why does the area of insect bites turn warm and hard?
gpt
When your body detects injury it widens the tiny blood vessels around the injured area to let in infection fighting/body repairing cells.  
These additional cells causes heat, hardness because of the increased pressure, and redness because of the blood.  
A lot of bites like those of mosquitos inject a little bit of saliva. The body treats that saliva ust being there as an injury, so it might be going a bit overboard with the response.