When an insect bites you, why does your skin swell up?

Dear Bettina,

Insects such as bedbugs, fleas, and mosquitoes poke through your skin with their drill-like beaks. They inject saliva into the bite. That keeps your blood from thickening while it is being drawn through the insect's narrow beak.

Your body reacts to the saliva by releasing a chemical called histamine (HIS-tah-meen). Histamine causes blood vessels near the bite to enlarge, making the area red. It also causes swelling. Histamine irritates nerve endings, which makes you itch.

The body removes the histamine and insect saliva after eighteen to twenty-four hours, and everything returns to normal. But the more you scratch, the more histamine that is released, and the more irritated the area becomes. Resisting the urge to scratch will make the bite disappear faster.