The short answer is: the itch is an allergic reaction to mosquito spit.

Yes, spit. Mosquitoes use a mechanism involving saliva to get as much blood out of you as quick as they can. A mosquito’s mouthparts are made up of multiple tubes. The mosquito uses one tube to slurp up blood. Another is used to pump saliva into your blood vessel. The saliva acts as an anticoagulant—preventing blood from clotting and thus making it easier for the blood to flow without disruption.

But, as it turns out, humans are allergic to some of the proteins in mosquito saliva. That means that even though the saliva is harmless in and of itself, our immune systems are hypersensitive to it. The body reacts to the saliva by releasing histamine, a biological substance that dilates blood vessels in order to facilitate the arrival of white blood cells, blood plasma proteins, and other immune system tools at the site of injury. In the process, however, histamine causes inflammation and itching.

The best way to avoid the itching is to avoid mosquitoes in the first place. And although mosquito saliva is harmless, it can carry dangerous diseases, such as West Nile virus, Zika virus, or malaria. So it is important to take precautions to prevent mosquito bites:

• Use window screens to keep mosquitoes OUT.
• Wear long loose-fitting clothing to keep mosquitoes OFF.
• Use insect repellent on exposed skin and clothing to keep mosquitoes AWAY.

Be sure also to empty any containers of standing water, which serve as ideal breeding grounds for mosquitoes to lay their eggs.

Read the original article on britannica.com.

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